[CrazyCarl461]

This doesn't seem to be allowed even if the bumpers go completely around the edge of the concave portion. The wording is subtle but it doesn't seem like any concave configurations will be allowed at all by way of:

<R07>A: Bumpers must cover the entire FRAME PERIMETER

FRAME PERIMETER is determined using the string-wrap method (thus spanning the gap)

<R07>M: Bumpers can't span gaps where there is no physical frame on which to attach