I have installed several and have found a simple method that has worked.  Set my canoe on a piece of carpet on the garage floor. Then clean a spot on the bottom inside of the canoe where I want the transducer with alcohol.  Place a spot about 3/8 inch in diameter and ¼ inch deep on the transducer bottom of GOOP or RTV.  Then press it into the location where I want it mounted and place a heavy (10+LB) weight on it for a day.  A trick to get RTV to set well is to put it in place then pull it off and exhale on both surfaces and put it back on.  RTV requires moisture to set and your breath has enough.  Usually these can be removed by working a flexible thin blade under them.