There was a company that made purpose-molded inserts for APC's. I saw 'em at Toledo in '07, but not '08. Too bad, I needed some.
They also made a nylon "clunk" tank with a slack bladder inside. Anybody recall the name?
On APC's, the intentation is precision-molded, but the holes are drilled off-center. So these little plastic inserts were glued in. Then you reamed 'em to size and ignored the drilled hole.