I personally don't know of anyone that uses them since, with the boats they build/assemble, the drive shafts don't have one fixed angle they go through the bottom at. Something else to think about is the drive shaft itself. What size are you going to use in a build? There are three common flex drives commonly used:

• .187(3/16") flex cable
• .250(1/4") flex cable
• driveshafts using a single "wire", commonly referred to as a "wire drive"

Each has it's own stuffing tube requirements so you would need to have butch blocks of different sizes as well as angles to meet your needs. I would guess that's why they aren't use much anymore. I personally have used epoxy with silica fibers mixed in for wood hulls and silicon in FG where the material was too thin to use the epoxy mix.

Note to Dana, your son-in-law does nice work.