Reddit reviews Hoffen Stainless Steel Outboard Motor Bracket Mount Heavy Duty Auxiliary Marine Boat 20 Hp, 2 Stroke (Black ABS)
We found 1 Reddit comments about Hoffen Stainless Steel Outboard Motor Bracket Mount Heavy Duty Auxiliary Marine Boat 20 Hp, 2 Stroke (Black ABS). Here are the top ones, ranked by their Reddit score.
I shimmed mine out to overcome the angle of the transom. Shim was like 2" at the top, and 1/2" at the bottom.
Not an endorsement, but this one looks like it does what you're looking for:
https://www.amazon.com/Hoffen-Stainless-Outboard-Bracket-Auxiliary/dp/B073VQYSV5/ref=sr_1_12?keywords=outboard+mount&qid=1558369280&s=gateway&sr=8-12
Extra long shaft (25") is a good thing. Tohatsu makes an extra long shaft 6hp.
15-20hp is probably going to be too much. You're not waterskiing, and you want to minimize weight at the extreme end of the boat. 1-2hp per 1000lbs is generally enough. My current boat has 2.3 hp for 3200lbs, and only twice in the last 3 years with that outboard have I really wanted more.