Bottom of plastic milk cartons for various size pots.
And as mentioned, recycled pallet wood for bespoke trays.
Jimmy

I also make use of various containers and make larger wooden ones for beans etc

Got to agree with Veggiechicken, the seed trays from Wilko are superb compared to the usual flimsy rubbish on the market, although they seem to be learning from Amazon, my last 10 trays came in a box big enough for a water butt!

Another vote for the wilko trays here, I’ve been buying the individual little square pots too. I think they’re 10 for £1 and seem much more solid than a standard tray insert.

I have a stack of the wilko ones too. Also a few my parents bought before they had me (they have lasted v well!), and a few wooden boxes which I cadged from the grocers begore they threw them out

I have the thin green plastic trays from the garden centre and also reuse plastic packaging trays, the ones that you buy mushrooms in are particularly useful as they are quite deep.

Same as Ted's Mum.
I use the blue mushroom containers.
Also have a lot of good sturdy green trays , half size of the usual ones.
I try to reuse as much as possible as I am on a mission to get rid of plastic.