I did the same thing on my Ribcraft. I go to the west coast of Lewis most summers, and being without an auxilliary engine isn't an option, given location, lack of other boats in the area, changing sea state (always a bleed'n swell in the Atlantic).
I don't have an A-frame, as my boat is stored in a garage, so I opted for a bespoke stainless steel light pole. This frees up quite a bit of space. The steering drag-link on the port side rules out fitting an auxilliary on that side. I mounted the auxilliary engine straight onto the transom. I used to have a Plastimo adjustable bracket on a previous boat and it was a complete disaster, it rattled like a bag of spanners.
I have a Mariner 4hp 2-stroke auxilliary. I opted for a long shaft, but it may be possible to fit a standard shaft instead. The benefit of the long shaft is that there's no chance of cavitation in a rough sea, it'll stay firmly planted.
I also fitted a stainless steel 'U' bolt on the transom knee. I use a mini ratchet strap around the power head of the auxilliary, then through the starter rope aperture and onto the securing bolt. When it's tensioned its rock solid. If you don't secure it there's a real possibility you'll damage the outboard, tilt bolt, etc.
Choice of auxilliary is key, with weight, service costs, and of course overall dimensions. Might not be such an issue if you opt for a bespoke fixed bracket extension. A Yamaha or Tohatsu 5hp 2-stroke would be perfect in my opinion. The Tohatsu M5B is 21kg, and the Yamaha M5CMHL is 21.5kg, and both have plenty grunt, but might be overkill for what you're looking for.