British standard trays (internal measurements 344 x 214 x 52 mm). A decent quality economy product
20 x standard size seed trays without drainage holes
The manufacturer states these are "formed from .54 mm thick recycled high impact polystyrene". 
These are not the solid rigid trays that you can buy. There is a degree of flexibility.

With a little careful handing, these can be used over and over again- wash them after use and keep out of direct sunlight when storing them.