Cabinz, a Uk community group. Have reused 2 metre Grp fibreglass skin / upvc core off-cuts from a composite door factory that would normally be landfilled as waste.
This is a fire, rot & rodent resistant material, that is very flexible when the grp skin is broken. It comes in up to 7 colours, with white on the other side.
Step 1: Stuck Inside Dry Wall Lining
The off-cuts are 44/45mm thick and fits comfortably within most dry wall lining steel c & i-stud / track that wall builders use. There is a few mm left for glue or silicone.
Screws or nails can be added for extra strength. I recommend a self tapping dry wall screw (with collated gun for speed) . The upvc core can be nail gunned easily.
The combination produces a much stronger material, as core is strong in compression & the metal is strong in tension
Dry wall lining builds the inside of partition walls & is very cheap to buy. Different configurations can produce I beams T beams or H columns
The last pic shows other offcuts such as "window cut outs" being joined to make an insulated wall panel.
These beams can be used to support insulation on an existing building like a garage or make a new single storey structures like a shed..
Contact your local composite door factory or shop, as they may be throwing the off cuts out, and may give them to you for free. If in the uk, visit http://www.cabinz.net we may be able to supply some at low cost.
Of course, other materials could possibly be used as a core, like wood, plastic pipe. perhaps even
cardboard tubes ?
It may be best to clad in c-stud dry wall on all sides if using weaker materials (Disclaimer: we have not tried any of these materials ourselves)
Dry wall lining metal is available cheaply all over the place