i actually tried quite a few different plastics at home depot ... i found that they worked for loads that were well packed in the 20-30 lb range ... ie where you could get by with frameless, but the plastic sheet added a bit of stiffness ... at the 40 lb mark i found the plastic concaving under the weight, especially if the load was not optimally packed
i also tried a few different woods such as fiberboard which had the same issue as plastic, balsa which was worked but was expensive, etc ... the best is ultra thin, ultralight with the grain running vertically
with what i have now .... ive carried a belay jacket, a double rack of micronuts, double of regular nuts, single of offset, set of WC zeros, set of camalots, set of link cams, 1/2 set of TCUs, extra #3/#4/#5 camalots, set of tricams, 6 quickdraws, 8 trad draws, 2 screamers, anchor tat, harness, set of rock shoes, 3 L of water, lunch, guidebook, 60m rope
for the vertically challenged ... thats something like 15-20 lbs of metal gear, 10 lb of rope, 7 lb of water, and 5 lb of other gear ... about 40+ lbs total
and i throw it in anyway i want ... and theres not a sign of frame collapse .... my knees however are another story =(
its all there except for the 3L of water. guidebook and lunch ...