mounting rod arch - acceptable backing plate?

[McGillicuddy]

I'm thinking I'll mount an aluminum rod arch on the Seafari this weekend. It has a 2" wide plate for the base which will be mounted atop the ridge cap immediately aft the side windows with 4 or five 1-1/2" x 1/4" s.s. pan-head machine screws with s.s. fender washers and nylocks on the underside. I was thinking I'd use 1/2" thick polyethylene strips(think cutting board) on the underside of the cap to better distribute pressure on the cap (keep in mind the inside width is only 1-1/4" and 4 or 5" deep so proper coring will be tricky at best). Is that foolish or can I do that with out losing sleep.