Pallet collars are pallet sized collars that are hinged box sides. They’re used for storing stuff on pallets.
I’d seen some on a plot, but it was only when I discovered one labelled as a pallet collar on the internet that I realised where I might find some. My local pallet yard sold me some at £6 each which was cheaper than me building a raised bed from wood, and definitely easier than the effort of me making raised beds. I used about 8 on my plot. Someone new joined our site and whilst showing him round my plot, he realised he had them at work in the engineering section. So he went and asked nicely and got given a load: A serious load! His source has dried up now unfortunately but not before he’d laid out his allotment how he wanted.

I’m a bit jealous as he got his for free – although eh did have to load them into his car and fetch them to site himself, which took several trips. He’s now got an excellent looking plot.
He’s filled his boxes with manure and has mixed in some coffee grounds he also gets from the café at work. His seedlings are growing in his plastic greenhouse and will be ready to be planted out soon enough.