Access ports are widely available, but contoured rims, not so much. What diameter are your current caps? I think you should just buy a whole new port including the rim.
Last summer lost a screw cap on Dart 18, ordered one at great expense from Windsport. cost more to ship than hatch cover, but picked up at West Marine base and screw top and top(clear so nice addition) actually fit! Screw tops/hatches come in a myriad of sizes ;when I had a Nacra5.7 I selected one from Murrays and they had sizes hooked up to each cat(not Darts)