Rob,
First, I agree about Humminbirds and would not go to the troube of installing one even if it were given to me. I have used the Garmin 240 and am now using the Garmin GPSMAP 168 and like it very much. I also have an Eagle Accura 240 and a Lowrance X-85. I think the Garmin 240 and Eagle Accura 240 are the best value, and like them better than the Garmin 160 because of the high resolution.
Maybe it's the epoxy you are using. I've never had a problem using the following method:
1. Lightly sand, then clean the hull area with acetone or lacquer thinner.
2. Sand bottom of transducer with 320 or finer paper.
3. Make a dam about 1/4 inch larger than the transducer with a bead of RTV.
4. I recommend using Devcon PLASTIC WELDER. When you have everything ready, mix the epoxy in a small cup thining with a few drops of alcohol. Thinning with alcohol makes the air bubbles disipate faster.
5. Pour the mixed epoxy into the mold and wait a few munites for the air bubbbles to disipate.
6. Set the transducer in place and put a weight on top. I find that a bag of lead shot works real good. Note: I do a lot of skeet shooting and reloading, so I always have #9 shot available. A bag of shot works good because it doesn't want to slide off, but anything that is fairly heavy and will stay in position will work.
7. Wait until epoxy cures. Note: Cure time will be a little longer because of the alcohol has to evaporate out. Temperature also effects cure time.
Russ