Cold frame

I don’t really have space to set aside for one, in the past I’ve used an old tin bath with a piece of corrugated plastic on top. Since then I’ve bought a couple of hard plastic cloches with vents that are handy for hardening of seedlings or covering crops from a late or early frost.
When they’re not needed they easy stack so don’t take up much space.

If I had space I’d go for wooden sided to give more protection, with angled top in full sun then id have the option to shade if i needed it.