I once rigged one out of a few pieces of scrap 2x6 and rigged it to run off the side of the boat. A creative individual should be able to make one out of a board rigged between the stern of 2 boats. this has the advantage of allowing a shortshaft motor to work on the lower tube height just before the boat rockers up at the stern.
In a pinch we once did this off an oar, between 2 boats, because the ranger made us leave the metal transom at the put in, since we didn't have the boat registered to run a motor. (the motor was well enough stashed that it never came up).
DRE makes motor mounts like the one pictured above, with a few more bends, so the tubing follows the top of the curved stern tubes, allowing better access to the cargo bay.
A chunk of frame tube or a broken oar work well s a tiller extension.
The motor can be raised or lowered by changing the air pressure of the raft tube, if you don't have one of those spring loaded height adjuster thingies.
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