My boat has a TM mounting thing welded into the body, so my install was straightforward, but I am using the same QR mount as you. One idea I had was to try the nut-sert or nut rivets, but I don't think those are strong enough to hold a heavy TM in rough water. If you could make a plate and tap/thread the holes, you could put all 4 bolts in the plate and guide it up from the bottom, then use nuts on top.
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