Different ways to mount for testing based on what kind of hull/engine you have. If you have a clear shot down the rear of the transom, use a piece of oak 1x2 or 1x4, mount the ducer on it and then c clamp it to the transom (far away easiest and best way to test it out). Second best is to mount a plate to an engine bolt that can be adjusted for testing. The last resort is 3M 5952 two sided tape. It's pretty good but getting it off can be a pain. Don't leave it on there for real long and it won't set up as bad.

Remember - idle speed only for testing so you don't detach the ducer and eat it with a prop.

I haven't used si much for fishing but I've used it a lot for underwater archaeology and geomorphologic data collection. Ducer placement can mean the difference between getting sub centimeter separation and not being able to see much of anything but ten centimeter blobs.