I’ve been saving old egg boxes for some time now, with the intention of trying them out as seedtrays. The idea is that I plant the seeds in them, the cardboard survives long enough to let them germinate and grow a little and by the time they need to be planted out I just tear apart the egg box, plant the modules and the cardboard decays by itself. I planted more than one seed per module with the idea of thinning them out to one seedling per module. Which is fine in theory, but the seeds don’t realise this and in one module all grow and another they all sulk, meaning that half the egg box is useless, but can’t (realistically) be detatched.
I’m not sure that the newspaper will hold together long enough, but then the egg boxes looked as if they might last a bit too long. You can write what the name of the seed on the newspaper though, and it’s tricky to write on egg boxes. I confess, we do also have some plastic seed trays (bought… I know, I know..) as well as pre-loved plastic pots. I also heard about someone using baked bean tins with ring pulls (ring pull is the bottom for easy extraction of the plant). So we’ll try them all and see what works the best.