After having a look at the use case, I would likely go with a modified project box, rather than your right angle gearbox idea. It sounds like you’re going to run into a world of pain, with some of the details.
You can buy project boxes of various descriptions that are waterproof. Some even have doors on them! If you took one of these, and cut a hole in the bottom or back (as appropriate) you could mount this over the valve. This could be sealed down and stsy in place. When you need to use the valve, you can either open the door if it has one, or remove the lid from the project box. This gives you access to the handle. Afterwards, close it back up, and the project box’s waterproof seal will keep the water out.