Good job on the buggy. I think you will be quite happy with it. The XL motors are still good motors to be sure - it's just that some people want high-end for more efficiency (cooler running). I still run a 10XL in my Revo and love it.
About the straps; I just use
this stuff for all my batteries so far and it's worked great. It's just a continuous roll of velcro. If you attach a piece to each end of the tray, it'll help keep the tab from bending in all but the most severe crashes. Even then, it should work. It's not GorillaMaxx straps, but it's easy and cheap to replace.