Lets here it for a fellow tinkerer. Creating an idea and making it work is the reward. THEN you get to go play with it.
I would suggest a spring out of a pumploading shotgun. Paintballs fit nicely inside a 12 guage shell so the diameter should be about right. If not they make 4-10 pump shotguns.
When you start slotting PVC pipe the diameter gets smaller. It is stressed during cooling in the fabrication process. You cut a slot, it springs into the space and closes the diameter. MY experience was with 2" PVC pipe to make a spring loaded Nerf launcher. Smaller PVC pipe MIGHT be different but I'd wager you will be working with metal or a solid tube of PVC pipe.
Two types of PVC work well with paintballs. 3/4" CPVC is the yellow stuff used for hot water that you can buy anywhere. It has a very good bore to paint match. Made my own barrels out of it... Just look down the inside. Some are smoother than others on the inside.
The other option is 3/4" Schedule 20 thin wall white PVC. It is hard to come buy because of local building codes. But if you live out of the city, you can find it. It has a fairly loose bore fit on paintballs and I would use it FIRST if I could find it. You might be able to slot it and still roll a ball through it.
You were talking about a detent. Lancer is a spyder knock off. has a piece of rubber. You could experiment with making your own. Take it out and eyeball it. Then the aftermarker detent replacements from other markers, spyders etc.
I'd guess you might not need it. If the clip is mounted below the barrel you are going to have some plumbing 45s and 90s to push those balls past. In fact, I would recommend some tubing instead. Have a vendor. It won't kink and slides right over a spyder feed neck.
Pondering some way to have a moveable detent in your clip that would open and close with each firing of the marker to let ONE ball pass. A rocker arm type mechanism. I really wouldn't worry about it too much until it becomes an issue. Just plan ahead so you have room to build something else for a detent mechanism. We could ponder it a bit. Maybe a detent in the tube and use the blowback air that comes up into the feed neck to BLOW the next ball past the detent. Blow between the next ball and the second ball. The air would push the stack of balls back a bit, Tend to hold them in place against the spring. If the Lancer has a feed neck hole to vent that blowback air out of the feed neck to increase ROF and avoid mis feeds, then MAYBE the whole solution would be to tape it SHUT and if it works, block it permanently. Make the hole smaller, etc.
Instead of notching the tube. I liked the cord idea and a solid tube. Pull the spring back with a rope. End cap with a hole in it would work fine. Put a notch in the side of the hole. Put a knot in the cord close to the front of the spring feed plug and just pull it though the hole and into the notch to hold it long enough to reload easily. The cord would gravity feed back into the hole with a little encouragement and hold it there with a velcro strap or something.. How about a small cork on the front side of the cord that just PLUGS the hole. Friction fit. A larger hole would be easier to gravity feed and push that cord back into.
Brainstorming and problem solving is my hobby besides paintball. e mail or post here and pick my brain. firstname.lastname@example.org
There, chew on those ideas a piece and then swallow or spit........