Plywood upgrade to match coefficent of friction

We are building our field obstacles out of plywood. Does anyone have any ideas on what we could add to driving surfaces to get closer to the coefficient of friction for what will be on the actual playing field? Thanks for any help you can give us.

We are thinking of varnish or shellac. Other option is covering with air duct metal.

We’re covering our plywood with 1/16th HDPE Sheets.