I bought some from here:
http://cgi.ebay.co.uk/10-x-Protector...4.c0.m14.l1262
However they are a snug fit on 50mm webbing, so you would not be able to slide the hook / ratchet through it if you needed to.
I would suggest you would be better off reviewing how you route your straps to avoid the tubes (I'm only a humble SIBber, but I think most RIBbers strap down at the bow eye and transom only), or use carpet offcuts (this is what I do on my SIB).
Cheers
Chris