Im not on here that much, so a little late picking this up. Are you talking about TT25 transducers like the ones Mrrcsound sell/ I suppose they all work the same, I've used these a lot, in planes mostly, and experimented with boats, so can offer some hands on advice
Firstly, you don't cut any holes to let sound out, as this isn't how they work. its all about the vibration
s. The centre ring is epoxied (that's the best way, they have to be permanent, but with some teasing they can be removed it required, rather than cyno) to the surface, which obviously needs to be flat. The thinner the material, the more sound, but its marginal, as the thinner you go, the less bass, or deeper tones. With planes, the best material by far is the epo foam, so when mounting into a ply or balsa plane they work best going to foam, then the ply, and the same will apply for a boat. 2-3mm is generally the optimum thickness. Remember, the area is going to vibrate, so use an area that can do this, the sides of a hull are ideal. The back of the transducers get hot, so don't cover
the back, the heat has to dissipate and once secure, make sure the unit can move in and out, its easy to drip glue in the wrong place and the whole thing gets stuck, it wont vibrate now, so won't do the job.
Also, its worth epoxy
ing the transducer to 2mm foam, epo that the ready to fly planes works best, its close density, the stuff that packs white goods is poor as its a more open density and falls apart. Once you have this, you can move it around by just holding it against the hull to see where the best sound is.
All this is relevant to the Mrrcsound transducers, I use a number of his sound units, so cant really comment on what you are using as I cant find that info on this thread (did a man read!)
Here are a couple of my model
s to give you an idea👍
so these are all Mrrcsound units, and both have two tt25 transducers either side ogf the hull and fuselage.
With the Mrrcsound system
s, you can use two tt25, if you want an additional two, then an aux amp is required
hope that helps!