> I tried something like muslin, but it deformed
Well, sometimes, you want the trial fabric to drape in a similar manner to the actual fabric, especially on curved areas of clothing. And the properties of the fabric will depend on a number of factors, including whether they are coated.
The same is true of tents, where the tensioned fabric stretches anisotropically, due to the warp/weft fibres. Plastic sheeting and paper don't behave in the same way, so, whilst they're okay for roughing out ideas, they won't give you the true panel shapes for complex or tensioned designs. Close, but there are wrinkles. That's my experience, anyway.
The classic example from clothing is the 'bias cut dress'; the panels are intentionally cut on the bias, and the weave stretches diagonally to the warp/weft so that the dress clings to the body, creating a very flattering look.
Interestingly, very few outdoor clothing manufacturers use bias cut panels, even on panels that could benefit from stretch...
For packs and other simple items, plastic or paper are fine.