To answer the question, The easiest to build, cheapest, lightest, fastest turning, most playable, etc etc turret is a frame and ALL netting. You need the visibility to survive and for player safety. You also need the air circulation and ventilation.
Make a dome or box out of PVC or whatever LIGHT.
Now here is the nifty part. Get an old office chair. Take off the wheels and bolt it to the floor or gunner platform. Attach a seatbelt. From the back of the chair, attach a post UP and attach it to the dome. You have an umbrella made out of netting on an office chair that turns with the gunners feet leaving both hands free to reload, aim and fire.
You will have to decide if you want the gunner semi standing or fully sitting on the office chair. If semi standing, you will probably want to extend the height some way such as a platform box or just get a swiveling bar stool, bass boat seat, whatever.
That is the concept. Now some givens.
You want the center of gravity of all the gear mounted in the turret to be AT the piviot point of the turret. Otherwise, the turret will want to turn the heavy side downhill when you are on a hill which is MOST of the time. That can wear out a gunner quickly especially if you have a HEAVY turret.
So when you are mounting your cannon and marker in the turret, think about using the air tank as a counterbalance in the BACK of the turret. You can even mount the air chamber, valve, etc remotely to counterbalance the weight of the cannon and marker. If you were smart, you would design it so you could shift components around to tweak the center of gravity and then make them more permanently mounted. Zip ties and then hose clamps.
Cheap and motorized are mutually exclusive words when talking about paintball tanks. Still, they are nickel and dime projects after you get the base vehicle. $50 can keep you busy all weekend. Cut back on you other vices and pay and build as you go.
Don't forget the $125+ you pay to buy tank insurance for EACH game. Extra transportation costs to get there.
If you want CHEAP I have an alternative that solves all those problems.
How does construction cost under $500 sound to you. Want to build TWO for the price of your base vehicle?
Build a Walking Tank. No tank insurance requirement. Mine goes INSIDE my Dodge Mini Van or in the back of my pickup and adds NO extra transportation costs. Don't have to buy a trailer. Pay for storage or mechanically maintain the base vehicle.
The only downside is if you have motor or transmission problems, they have to call 911 instead of AAA.
The reason we see so few walking tanks is people can't make them LIGHT enough to have fun playing in. I have solved that problem and until defeated armor vs armor happen to own the "worlds most dangerous paintball tank".
Undefeated in 5 years by opposing armor. It is the RPGs and the grunts that are the threat.
My walking tank is a Mech Warrior and is mounted on a backpack frame. I CARRY it on my back. Minus the paintball gear the shell weighs 20 pounds. Battle ready with all the gear 65 pounds. I'm 60 years old and can WALK around a paintball field all day in it. If I can do it, ANYBODY can.
It could be lighter still. It also carries a 1 RPS autoloading Nerf Cannon. Something you probably won't start off with. That adds about 6 pounds over a standard breech loader.
You have HUGE advantages over motorized tanks. Given all are using Nerf Rockets and PVC barrels the range and accuracy is the same. YOUR advantage is SIZE. Every motorized tank is at least TWICE the frontal area as my walking tank. A pickup/van sized tank it 4-5 times the frontal area. That gives me a two to one or BETTER advantage in a head to head engagement.
In paintball tanks, Bigger is NOT Better! Smaller is better. I pretty much took the smaller is better concept to an extreme. Smaller is LIGHTER too.
In addition, I rarely go head to head vs armor. I have mobility. Many fields do not require walking tanks to stay on tank roads. No safety issue if I step on someone. I did and he was happy to surrender. Stepped on a Kid in a ghille.
That allows me to use TREES, Bushes, Cars, Bunkers, Birms ANYTHING for cover when going against armor. Anything that reduces the target area for my opponent. I can step sideways and dodge incoming Nerfs from a long distance. Try that in a motorized tank!
I ambush, use cover, hit and run, against armor. Avoid going head to head. Same tactics as the Sherman vs Panzer in WW-II. A Sherman could only go head to head against a Panzer ONCE....
That also means I can RUN with the GRUNTS in close support of missions. That is a HUGE tactical advantage over motorized tanks.
The trick is building it light. I use a special, extremely light, very strong and somewhat expensive material. Polycarbonate Twin Wall sheeting. A 4x8 sheet weighs 7.25 pounds and cost about $80. Expensive but you don't USE very much of it. Cardboard made out of the material they make safety glasses and bullet proof glass out of. It takes point blank paintballs at 300 fps without a dent.
It is assembled with 90 degree polycarbonate profiles and glued together. Those profiles are about $3 a foot but again, you don't use much.
It has NO FRAME other than the backpack frame. It is a mono body structure. The "armor plate" IS the structural material of the body. A third of it is netting anyway.
Now when I said $500, I included the cost of a cannon in that.... Material costs is UNDER 350..
Something I would like to see play is a tank like mine with NO Cannon. Just a marker only. That would put it in the 30-35 pound category. Picture 3-4 of them scurrying around a field together raising havoc..... I can hear the other side whining now......
I have pics and How to files of this tank build. Fabrication techniques etc. I share all my stuff to help the Sport evolve. Anybody can just ask for the info. E mail me at firstname.lastname@example.org