If you go for the sump fixing option, you need to ensure that there is solid GRP between the txr & the water, no voids, foam, wood etc. It needs to be transducer-solid GRP-water. You can get transducers specifically for the job, as someone already mentioned, they are mounted in a tank of antifreeze bonded to the inside of the hull, this is what I have on BP
. If you don't want to got to that extent, then experiment with bonding an ordinary txr. You can try various mounting positions (ooer missus) by filling a bag with water & sitting it on the proposed fixing spot, put your transducer on top of the bag & see if you get a clear echo, the boat needs to be on the water at this point, obviously
Alternatively, you can use KY between the txr & hull to test the position, if you're happy with the echo, glue the txr in that position. Personally I'd use epoxy to fix it, but others have had good results using mastic. Time spent now getting it right pays off later, it took me over a day to get my txr positioned & fixed, but I get a faultless bottom trace at all speeds up to WOT, 40kn ish.