How about some epoxy between your mounting blocks and the tube with a couple of 1 5/8" fasteners to hold it in place until cured. You should be able to get inside a 12" pipe with an impact driver and get the fasteners flush with the tube to prevent snags. If you prep the pipe surface and the mounting block correctly, it should be stronger than the mount to the trailer frame.
$77 is a lot for a 12" cap, but it's not unreasonable when you look at pricing for fittings in that size. I think that's why a lot of the sail tube designs on this site use an alternative method of sealing the ends.
After hours shopping at the local housing development maybe?
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Jeremy Hales
1974 SOLCAT 18
Salem, OR
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