Never tried the plastic frames, but I once built one of those "hellcat" alice packs using the "molle II" ruck kidney pad and shoulder straps. It still sucked, but it was much, much nicer than the original ALICE equipment, and worked as okayish as can be expected with a plate carrier. Still they're nothing but a money pit with little to no payoff.
Got the old 3 color desert stuff off of ebay for super cheap and dyed it. Came out kind of a blueish green, but it worked fine.
Seriously though, if you can save pennies and swing for a used/sale price Mystery Ranch, Kuiu, Kifaru, Hill People Gear etc. They will last for a long, long time and your body will thank you.