For fit, the standard shell is the best no question, however I run/ran my Vykal with no issues only with Velcro. See the pics below you can see that I use velcro most of the way round the chassis edge and it's fine. You could also use a body clip as per B4 at the front (which I don't use) or mount drilled cap head screws on the chassis sides as Ellis does.
To note, my Vykal was pre production and didn't have cut lines so I cut at the bottom fold in the lexan, this might explain some extra clearance. Even with the stock X6 shell I now cut below the cut line for extra clearance with 5000mah+ lipo.
the cell brace does rub the shell hence the strategically placeed Vampire stickers
