How about cannabilising an old big floodlight housing?

Most have a locking adjustable tilt mechanism at each side, where it
attaches to the bracket.


Or not even the floodlight housing itself - just use both sides of the
bracket.

Remove or hacksaw the arms off each side of the bracket, and assemble them
directly together, with the serrated surfaces facing each other.

We've got some big marine floodlights here with some beefy aluminium
brackets that would certainly fit the bill.

--
Ron