My boat has a cheap PiranaMAX 150 LCD depth sounder by Humminbird. A few months ago on a bad launch during rough weather, the transducer mount struck the concrete boat ramp. Needless to say, it broke away from the boat. I tried gluing it back together, with limited success. Since then, I have purchased my 3D printer and whilst out fishing a few days ago watching it flail around behind the boat, the genius penny dropped to make my own! Actually, no it didn't. The first thing was to spend 3 hours searching for a 3D model to print, alas there were none. So here we are.

Replacement brackets are quite expensive at AUD$51, when I factor in that the initial purchase cost of the ENTIRE sounder kit was only AUD$65!!

I'm currently printing out the "Prototype" mount to see how it fits. I say prototype, because I forgot to add the nut recesses and found a few hollow areas whilst printing. This newer version should work just fine. It will also be a lot stronger than the original Humminbird mount, as I have added an extra support leg where my original one let go. They also used really thin hollow plastic. I am still using all the other mounting hardware.

I printed in PLA with a BRIM, as my printer is not the best when the airconditioning system is running, likes to curl everything.
I also used supports, but it only filled the main bolt holes so unnecessary.
25% infill, entirely up to you. My 15HP motor won't break that off with the water drag :)
Used about 10100mm of filament.

This thing was made (badly) with Tinkercad. Feel free to edit it to fit your sounder at:
https://www.tinkercad.com/things/4k4Xy3a3c6i
(Edit: Oops, accidentally marked as private. Go for it now, sorry)

Rostock

Rostock MAX V2

Doesn't Matter

Doesn't Matter

0.2

25%

You can use less infill if you wish. I'm hoping that 25% should stay put for a while.rnEdit: Still hanging on the back of my tinny just fine.