Try this: www.hugedomains.com/domain_profile.cfm?d=foreverflying&e=com
Adaptors are at the bottom of the page.
You'll have to email him for price and availability
Absolutely ! Good idea I got some Tuttle fins but need a powerbox opening for my smaller board l recon it should work hey its not rocket science! 
I have never converted a fin, but I made up a few molds to repair a few powerbox fins when I ran them aground and broke half of the fin.
I used plaster base-coat in a chinese takeaway container to mold a complete powerbox head. Once it is set, you can then mix up some epoxy with chopped strands, and then insert the remaining head of the broken fin, and let it set. When its done, you break the plaster. You then need to sand back the head to get the shape spot on, but otherwise it seems to work.
If you are converting tuttle to powerbox, the same process would work, as long as you trim down the tuttle head to fit in the space for the powerbox.
