I'm with Doggy on this one, just screw your gap filling piece to the top of the transom.
Wood, or ali box section would be good, but make sure you coat the screws in sealant before screwing them down.
If it can be slightly less easily removeable it would be good to bed the whole thing down on some Sikaflex or similar.
I would also make it a little thicker than the transom on the inside to provide a small lip that will stop the engine slipping off if the engine clamps become loose.
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