My ROV projects are based on the ServoCity aluminum beam system (see https://www.servocity.com/structural-components/channel/mini-channel). As such I have adapted the mounting system on the Thruster to match the spacing of the beams.
Two bolts are slipped into the spaces in the main housing. These are a tight fit, but if they are loose you can epoxy them in place. The bolts are then passed through the beams and bolted in place.