I too have heard a lot of negative reviews on Cushman Custom Creations. Let's just say it's enough that I would spend the money on one of their bodies if it was going to cost me more than $5.
If you're looking for the cheapest route to create a custom marker, your best bet is to get a dremel and DIY using something like an airsoft shell for your base.
Like someone else said, if you're wanting to drop some money, Engler does some ridiculously amazing custom work.
I've built a custom marker or two in some free time with basic tools in my garage and there's nothing quite like taking it to the field and showing it off. When people ask where in the hell you got that from, it's FAR MORE SATISFYING to say you built it in comparison to saying you bought it somewhere.