I believe the problems with the Velcro, at least on my personal bags is that the front puckers and you have to squish the lid to conform with the puckered Velcro. I believe the design was supposed to just use the Velcro to seal the back and front of the lid not to actually hold it down. Of course the lid sags too.
There is virgin polyethylene and polyethylene that has recycled materials in it. The virgin stuff comes in smooth and textured and the stuff with recycled plastic in it comes only in textured. I'm opting for smooth to match the already installed back panel.
PE is very forgiving in low temperatures whereas ABS or polystyrene isn't. PE remains flexible and don't get brittle. That seems to be the ticket. We all don't ride in 80 degree weather all the time. PE is also more expensive, but you guys are worth it.