What type depth finder do you have?
You can also mount the transducer inside the hull if there are no bubbles, etc.. between it and hull. Can test using water or oil and can use epoxy to mount (a bit permanent tho).
A good price performance option is Raymarine ST40 series. Small footprint on both inside and outside of bulkhead. Can get the Bi-data version if you also want speed.
I have had a fishfinder bulkhead mounted and because it came with the boat with a transom transducer tried the epoxy inside hull method. Worked well. The transom mount was a disaster ... dont do it.
When the fishfinder died I replaced it with the ST40 bidata. It is a real treat!