Life your undermount watertank up beneath the tray, behind the guard. Fasten to the tray using M8x25 bolts, a washer each side and a nyloc.

Ensure that the correct bolts and washers are in the correct orientation

Make sure your watertank is in the correct position by ensuring the gap between the guard and the tank and the bumper and the tank is equal. 

Fasten all bolts.