Were you wanting a patent style TC hawken style breechplug with hook or just a regular breechplug with a hook at the rear?
If a regular hook is what you seek get yourself a 5/8 x 18 grade 8 bolt at Tractor Supply, Farm and Fleet, or eBay and install it in your barrel, saw off the excess and match it up to an existing hooked breech with a grinder and a file.
Shouldn't be to hard to do especially if you have a hooked breech plugged barrel to measure and copy from. If not you can remove the tang to get it ground/ filed correctly by using it to check for fit.
Bob