Every since I noticed rectangular shaped PVC water drain downspouts I have "enjoyed" the thought of using PVC as battery pack containment shells.
PVC pipe comes in various sizes, shapes and thicknesses.
Pipe, rectangular gutter downspouts, square fence posts, flat sheets etc.
PVC can be bonded with PVC primer and cement or "melted" together.
Durable and waterproof, if shaped to desired form and dimensions, might be the perfect material!
A heat box (oven) must be constructed. (Don't let your wife or mother catch you using her oven!)
An old electric skillet w/thermostat, heat gun, possibly even a hair dryer might work as heat source.
Also important, as determined by sheet plastic forming experiments, is a circulation fan to evenly spread heat.
Using 2 internal forms with spreading capability is the simplest method of attaining desired shape and size.
External shaping, as in form below, might be required to attain precise shape. (See example below)
"As for building a pack ...
I have toyed with the idea of reforming PVC pipe.
Heating to the point of softness then inserting a desired shape-form, then allowing to cool, then removing "form".
In the case of your pack:
a house shaped rod and a box shaped rod
Place both inside pipe
heat till soft
with wedges, spread house and box to stretch-form to desired size.
PVC (Polyvinyl Chloride), like most "plastics", can produce harmful fumes if overheated or burnt!