I've done this before.
Make a spanner out of plywood that fits the profile of the nut you are trying to remove. It can be just a 100mm round bit of ply with the profile cut out of the centre that fits the 6 profusion on the nut. Helps if you have a jig saw.
Then make a square sided tube (internal measurements of maybe 70mm x 70mm and maybe 300mm long) that you screw to the 100mm piece of ply. Now you have a wooden tube spanner. The inside of the tube has to be bigger than the depth sounder. From memory mine is about 60mm in diameter.
It a bit of a project but more than likely will work.
Another way is to try get a block of timber (best to use hard wood) about 40mm x 40mm x about 300mm long and drill 2 holes (different sizes maybe 3/8" and 7/16") through it that line up with profusion on the nut.
Now put the 2 drill bit back in the holes (sharp end in the timber) so that that they stick out of the timber like (blunt) spikes. Leave enough length sticking out on the drill bits so you can place the timber on top pf the depth sounder and the drill bits reach down into the profusion on opposite sides of the nut. Now just turn the timber.
You could use a wider piece of timber that would allows you to drill 6 holes and thus 6 drill bits sticking out that line up with the 6 protrusions.
Never give in
(I said to use different drills bits for each hole as most people don't have heaps of the same size drill bits and the drills need to be firm in the holes)