Can you supply a photo? Do an internet search for a clamp on shaft collar of the size you need? On place might be Amazon in the link below.
thanks so much Phil, I'll order nowWe have them here:
Hunter 49 rail clamp, davit, arch, bimini clamp 2 inch rails
Hunter 49 rail clamp, davit, arch, bimini clamp 2 inch railsshop.hunterowners.com
Looks like they're out of stock but we can get them in a few days.