Those same GTB plates will fit the XBR. Just doesn't use the center screw.
With a little tinkering you can make the XBR pretty thin actually. I cut the pins to make the two boards closer with mine and cut the case all up to make it thinner. Just needs some tape between the boards to keep them from shorting out and it works great for 17.5/13.5 in 12th. Saves about .3 ounces over the GTB 4cell.