Bulk substrate and the tub
A low nutrition bed that holds water, inside a box that holds humidity. Cheap, bulky, and where most of the volume of a grow comes from.
Coconut coir
The body of the bed. Holds water without holding much food.
Why: Low nutrition is the point: the mycelium is already fed by the spawn, and anything richer feeds the competition instead.
Improvised: Coir bricks from a garden centre or a reptile shop are the same material, usually cheaper by weight.
Vermiculite
Opens the bed up and holds a reservoir of water in its layers.
Why: Coir on its own packs down. Vermiculite keeps air pockets in the bed and releases moisture back to the surface as it dries.
Improvised: Horticultural grade from a garden centre, medium grade, is the ordinary version of this.
Clear tub with a lid
The chamber the bed lives in.
Why: Clear so you can watch without opening, and with a lid that actually fits, because every gap is a humidity leak and a way in.
Improvised: Any clear storage box of a sensible depth. Deeper is not better, it just holds more air you have to keep humid.
Liner or paint for the sides (optional)
Blacks out the walls of the tub.
Why: Mycelium fruits towards light and open air. Dark sides push it to make its mushrooms on the top surface where you can reach them.
Improvised: A bin liner taped to the outside does the job as well as paint and comes off again.
Tape for the holes
Covers the air holes with something that filters.
Why: Open holes are a direct route in for spores and for flies. Taped holes still exchange gas, just slowly and cleanly.
Improvised: Polyfill stuffed into each hole is the older approach and works, it is simply harder to keep tidy.
Gypsum (optional)
Mixed through the bed, it buffers pH and firms the surface.
Why: Optional, and the effect is modest. It matters more if your water is very soft or the coir is very fine.
Improvised: Leaving it out changes little in a first grow.
Large bag or bucket to hydrate in
Somewhere to pour boiling water over the coir and leave it to swell and cool.
Why: A brick expands to several times its size, and a container too small means half the mix on the kitchen floor. Covered, it also cools without collecting whatever is in the room.
Improvised: An oven bag inside a bucket keeps the hot mix clean while it cools, which is worth the small cost.