They were fabric with a slat screwed to the bunk base. Came perhaps 8" above the cushion. Can't recall ever having to use them, so can't say whether 8" was OK or not.
Sea berths I have slept in had a solid wood facing coming perhaps 3 or 4" above the cushion - just enough to catch a hip or shoulder. No complaints with those. They made me feel secure enough to get to sleep.
Another that has worked well for me is adjustable pipe berths. Hinged at the hull with a tackle system inboard - just adjust for level before you climb in. But certainly not suited for sleeping (if you can) while tacking upwind.