Inspiration
In November I watched a short documentary about a guy who grew pearl oyster mushrooms in his backyard. They used pallet boxes (half of a europallet, 60x80x20 cm) as box to keep the substrate the mycelium feeds on in. Since I really enjoy (foraged) mushrooms and had the raw materials lying around, I opted to build it myself. This also had the benefit of using what was available and not just consuming, i.e. buying a pallet box.
Preparing the raw materials
I had 4.5 m x ~ 25 cm wooden spruce planks at home. My plan was to cut those into 2 m segments, then trim the edges down to 20 cm and then cut them into handy pieces, following the dimension of half a pallet box. This is what they looked after cutting them with an electric chainsaw to around 2 m:

You can see that the edges are still not straight, because that's how they came out of the sawmill.
Once that was done I visited a family member that had a crosscut saw, a table saw and a band saw; all that I would need. First we trimmed the edges of the 2m planks with the table saw so they were somewhat straight; then they were flipped and the other edge was cut straight, and their width cut down to 20 cm.
After moving them over to the crosscut saw dividing them into two 60 cm and one 80 cm was fairly easy. When cutting the 2m planks from the 4m ones I calculated with extra offcuts, so I got little waste overall and could use the whole length to get my desired board.
This is what the cut pieces looked like:

Assembly
I packed up my planks, now nicely cut to size, and I went to a hardware shop and bought hinges and screws. Assembly was fairly easy and fast: screw a hinge to a corner, hold the other plank onto the hinge so that the corners of both boards touch, and affix the hinge.

When this was done, the frame looked like this:

As last step I drilled 10mm holes more or less random in the middle of the box. This is where the mushrooms will grow out of later and can be harvested.

Closing thoughts
This was a fun project I finished in a day. The hinges have the benefit that they allow the box to
be folded up lenght-wise:
This allows for convenient storage. Since it's too cold outside right now, cultivation will have to wait
until spring. This also just needs mycelium one can just buy, and some material fungus digests.
They can also be fed coffee grounds, and harvest of the fruit body is possible circa every two weeks.