Looks alright over all, you need to radius or chamfer those back corners though. Ditch the HDPE over the hardox, if you have 6mm Hardox you won't need it.
Don't use HDPE for the drum whatever you do, it will turn to a black twisted mess the moment it hits anything. Any of the good Alu grades will to: 7075, 6082, 6061, 2014. Check out 720's battle against NST and look at the hole HST made in its 7075 Alu drum! Now imagine if that was HDPE; it would probably destroy your weapon.
GR02's are no longer in production so you would either need to go Ranglebox or Banebots, either of them will do fine though the Banebots are much lighter but more costly.
All this said, Jamie, Dave and Guilherme are the people to go to for proper drumbot advice, they know their stuff.