British standard trays (internal measurements 344 x 214 x 52 mm)
10 x standard size seed trays without drainage holes
The manufacturer states these are "formed from 75 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 handling, these can be used over and over again - wash them after use & keep out of direct sunlight when storing them