In my Commander, I had a mechanical speedlog, and it used a speedometer cable to drive the indicator head. When the bearing in the skeg died, I removed the skeg and glassed over the hole, after grinding it a bit to give me a reasonable bonding surface. I drilled a hole forward in the bilge section just ahead of the mast. It works but I do have to pull it every so often to clean off the stuff. I thought if I needed a depthfinder, I could mount a combo unit there and be reasonably accurate.
Glassing up the hole in the cockpit wall was a bigger job and I have not been able to match the color of the wall.
If you want to go through a flat area, you are looking for something under the berths probably, The issue is that even when motoring, you are not going to be level. In either case you want an area that is accessable and not one that is going to be hit by things being stuffed in a storage area.
John G.
Valhalla
Commander No 287