Reduce the weight of a wooden box to carry it on the back

What a variety there is. That is somehow creepy. I’d rather not give any tips here.

ah I can’t help it.

With Wedi boards (Jackoboard etc.) you might be able to keep the box really large and instead use thicker boards to additionally stabilize them with long screws besides the adhesive.

The thing with aluminum/sheet metal is a good idea, the problem is that the places where this box is used do not allow aluminum/sheet metal etc.

Covered with a 2mm wood panel, it does not stand out. "Natural" materials will not help you anyway.

I can guarantee you that all of this happens on a voluntary basis for a partner costume.

I have also thought about Wedi-Platten before, but I was told they would never hold up if I use them as side walls.

They are available up to at least 5cm thickness. Then they can support my 86 kg (I even believe in 3cm).

I was also suggested Paulownia wood, but unfortunately I am not familiar with its stability