We've used "shoot through hull" on a couple of boats. We made a temporary with a plastic container sealed to the hull filled with water. When the spot tested good we made a level platform on the bottom with epoxy and mounted to that.
Will not work
with a cored hull.
Will not work well or at all with an air pocket.
Try another spot and let us know how it turns out.
We made a temporary once using a PVC pipe
with the sounder line running though it. Lots of fittings available to make a bracket and glues well. Use gray electrical
conduit as it has UV inhibitors built in.