I don't think that plastic bunks will support the weight without bowing in places from constant pressure. Then the boat is supported in small areas and not the whole bunk. Just my opinion. Pressure treated comes in different grades. Some are made for above ground use, and others for ground contact, under(posts) or on the ground. The more heavily treated should be the best you can buy, and then put the slides on if you're inclined. They make unloading an aluminum boat a breeze. Pay attention to those warning of dumping the boat on the ramp. Happens in a flash. Saw a friend that had them on his duck boat trailer dump his on the ramp. Nothing was hurt, but could have been worse. Don't unhook til the boat will slide off into the water. Kinda fun to watch though.
Creativity is just intelligence fooling around