You don't need HDPE mounts just some hose clamps. Drill 10mm holes in your base plate and wrap some insulation tape round the gearboxes so it has something to bite into. I've run a few robots like this and never had any trouble. Hose clamps are a couple of quid each from halfords or amazon etc. See picture below. Also makes it extremely easy to remove a blown motor ie. 1 min or less.