Yeah, countersinking metal is super easy. I have my screws sitting a 1/16 of an inch below the surface. No hang ups at all.
I'd recommend skipping the HDPE and getting UHMW Poly instead. HDPE gets really dry and slow. UHMW is a little more expensive but it's slick and durable. Mine's 1/4" thick. You'll have to round the edges and make sure your edges are completely detuned and rounded over because you'll def catch if they're square.