I was going to say that ought to be possible, but getting the reinforcement wood into the right shape and into position might be tricky. I think pre-drilling the pilot hole in the block would help make it obvious when it's in the right spot. Other than that, patience and some metal coat hangers ought to do it.
It's not as tricky as it sounds. From memory, as laid out in the how-to thread, it basically goes something like this:
Materials: A few feet of baling wire and fishing line, glue, a block of wood (doesn't matter what type), a small wood screw.
Tools: Drill & bit, screw driver, band saw or jigsaw, or coping saw to shape the block.
> Use the bass horn itself, as a template to trace the shape for the reinforcement block.
> Cut out the wood block with a band saw, jigsaw, or scroll saw. The block only needs to be an inch or two in length, and a couple inches wide where it contacts the inside of the horn.
> Drill appropriate diameter hole in bass horn where you want the strap pin located.
> Drill same diameter hole
all the way through the wood block (front to back), starting at front of block where it will meet the hole in the horn.
> Feed baling wire through hole in the bass horn, and pull it out through the pickup cavity, leaving a few inches or wire sticking out from the hole in the horn.
> Attach a few inches of fishing line to wood block by running it all the way through the hole in the block. Secure the fishing line at the
rear of the block with a small wood screw.
> Attach the other end of the fishing line to the baling wire lead.
> Insert the wood block through the pickup cavity, and pull it forward into position with the baling wire. Glue can be applied to the block before pulling it into position, or through the hole in the horn, with a hypo needle.
> With the wood block in place (with glue applied), screw the strap pin onto the horn, and into the wood block. This will hold the block snuggly in place while the glue dries overnight. You can keep tension on the fishing line to keep the block in place while screwing the strap pin on.
> With glue dried, remove strap pin, cut off the fishing line, reinstall strap pin. Voila.