I use a MinnKota with a 48” shaft on my 18ft boat and it works fine. I don’t have a removable mount. But I don’t think the foot print is any bigger with the removable mount then it is regularly.
This is one type of removable mount:
I was able to use stainless steel bolts with nuts and washers underneath. I have no backing plate and it seems to be holding well after two years of use and trailering. I did place rubber washers under the metal ones because of the uneven fiberglass surface. If you really can’t access the area below the motor. You can use rubber isolator bolts. They expand when tightened, much like drywall anchors. They are available most places trolling motors are sold.
You can see my setup in these photos.