well, let me offer what little I think I know-

Royalex is a vinyl/ABS/ABS foam sandwich and somewhat succeptable to solvent damage...
so I think G-flex epoxy would be a good bonding choice.

For the D ring patches, I think a heavy grade of PCV cloth, 40oz mebbe?.
Great supplier here:
http://www.mytarp.com/tarp-fabric-cut-yards.aspx

The rings could be attached to the patch using the same material and G-Flex or H-66. Then G-Flex for the patch to Royalex. Untested, unproven by me...just based on what little i know...