I've successfully used a transom on the rear of my NRS cat frame, made of two pieces of 3/4" plywood, u-bolted to the upper frame crossmember. With a long shaft motor, it's a quick and easy installation, and you don't need a "jackass" or other type of motor lift device. Might work with a short shaft -- I just never tried.
On my larger Jack's El Tigre Grande 20'/30" tube boat, the frame sits so much higher that I did go to a jackass. I had a small welded adapter made from heavy aluminum angle, which dropped down over the rear yoke crossbars. [See attached pictures]
Interestingly, on my Grand Canyon trips with that boat, I found that a stainless steel jackass (the one in the picture) flexed quite a bit. In contrast, one that my buddy on that trip (with the almost identical boat and motor) had a jackass made of heavy aluminum angle material did not flex. Sold my stainless model on Craigslist, found a used aluminum one on eBay, and the next Grand trip I had no flex at all. But that was with an 18 HP four stroke -- if you are using smaller, lighter motors in less aggressive water, then flex may not be an issue.
Hope this helps.