You got 4 holes there, so I would try to take advantage of all of them to improve stability.
A block of the correct dimensions drilled with two holes and two bolts through them would be simple enough to make even with basic tools. You could use a piece of wood and attach the appropriate thread for your tripod head to it in a number of ways. That would be the easiest.
If you could get a piece of rectangular aluminum tubing of the right size that would be pretty easy too. Even if the size isn’t exactly right a couple of washers would make the fit. Hardest part would be drilling the holes accurately for a good fit.
Of course you could always call around to local machine shops and have something custom made if you don’t mind spending.