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I've always made my own motor mounts as well.
Usually I make them out of aluminum (aluminium for those outside North America!). Basically I just cut out a rectangle of aluminum, bend approx. 1/3 up at a 90 degree angle (or whatever you need to match the angle of your prop shaft), and drill three holes in it, one for your output, and two for mounting screws.
Aluminum works well for this as it is very easy to cut with a hack saw, files easy, won't corrode too bad, and may help keep the motor cool. I usually glue the mount to the hull with something like silicone, as you can remove the mount at a later date by sliding a blade between the mount and hull.
You may need some short tubes to act as spacers between your mount and the motor, as the screws are back aways due to the gearbox.
Of course there are a lot of other ways of making mounts as well, I'm sure you will get some other ideas by poking around the site.