I have several of these a medium, fold-able wire dog crates approx 24"L x 18"W x 21"H.
Got them cheap(~$15-20) at flea markets and they are a great tool for many situations.

Add 1x2 wire to the bottom and put tray underneath crate to observe poops and keep feet cleaner.
Also easy to swap out trays for cleaning without handling bird.
If 1x2 wire is carefully installed, tray can still be used inside crate if needed.
Tray can also be used on top of crate for some weather protection if outside.

An ill bird isolated for observation....look closely to see how I added the cage wire to bottom.

This bird was in crate for a week to treat impacted sour crop.

Cup feeder works great and nipple waterer hangs outside crate held with mini bungie cord,
Being used here as a broody breaker crate. Chunk of 2x4 for a 'roost' was added to crate floor after pic was taken.

Close up of HN screwed into smaller hole just far enough to seal threads and some left sticking out to clip onto crate wire. Details on nipple installation here:

These were set up to photograph birds for sale.
Also use them during slaughter of young cockerels and/or old hens.
I usually 'catch' birds off roost at night after dark to put them in crates.

Also used as a safe house for integrating different age groups of chicks.
Later I removed crate door and reworked the front wire with one corner bent up to let small chicks in and out but older birds couldn't get in.