Viking, the biggest difference is Starboard is a harder, denser material suitable for weight bearing. HDPE should work for your amp board just fine though as long as you use like 1/2 or so. Once the amps are attached they will lend rigidity to the structure. As long as the board is mounted in a vertical position you won't have any problems. Just remember as HDPE is softer than Starboard you should back all screw heads with flat washers.
It should be a perfect fit for the false front on your sub project as well. also remember not to lay it in the horizontal position with weight on it or it will bow in long expenses.
2007 Mobius LSV