We have the same setup, couldnt be bothered to mount a proper switch and waterproof it all properly and this works really well for us, easy to turn on and off. Mounting points on the battery are using clamps like jumper leads, have tested going over big waves ect so long as battery is secure and doesnt move then the clamps wont budge, we havent had the terminals come off once since I installed them and all is good! Obviously you can strip and attach the wires permanently to the fittings if you want it to be perfect, but this overall was a much cheaper solution as a waterproof switch was about $50
Also the best tank to use I found was a beer barrel bought it online off feebay for 35 bucks had a lid and inlet pipe already fitted, just drilled holes in the top of the tank for water to flow out