So I have a spare Fusion Canon that I want to attach to CW Megatron's forearm. Issue is I can't hollow out the gun to insert and glue a 5mm post securely because that is the also the battery compartment.

Some possible ideas I've come up with are to skip adding the post, and sliding in these thin plastic panels I have (after cutting to size of course) into the railing where it would attach to MP Megatron and making a keyhole shaped groove/hole/cutout in the plastics panels to use the original red-hinged-mushroom peg that is on Megatron's arm. I don't really want to go this route as I want more freedom in range of motion.

Another idea I had was to use a 2x2 Lego brick and somehow affix a 5mm post to the underside of it. That would connect to his forearm, then using a flat 2x2 Lego brick and attaching that to the bottom of the cannon and then just connecting them. This seems doable but might create too much of a gap between the Canon and arm.

Any (probably better) suggestions?

The caveat is that I want to avoid modifying the forearm - and the Fusion Canon (though I'm more ok with that).

Thanks in advance!