To be honest, I highly doubt the extra 2.8v will make a huge difference to your motor life. I run a 5s 60C lipo with about twice the mAh, with four 18v motors and a brushless for the lifter. Since you're only running two motors and no weapon, I think that battery should be fine.

You say you're using polycarb for a template, but why not just print or draw it onto a piece of paper and (temporarily) glue it onto the HDPE? I'd think it'd be cheaper and easier to cut through. I know you're not cutting the poly itself, but I expect you'll rough the edges up if you're cutting along them.