Recycling old plastic packaging for seed trays

I've ordered tons of vegetable seeds and now have to think about planting them all, compost went on sale in Somerfield's yesterday I got a bag for £1.29, but wasn't a sign of a seed tray insert anywhere, I went round a few other shops today and still no luck. So instead of buying seed tray inserts I've decided to collect up yoghurt posts and use them to grow the seeds, I'll punch holes in the bottom and place them in the plastic trays you get mushrooms in to keep them from falling over. Not only does it re-use plastic it saves money too. When I've got them all potted up and some shoots are starting to appear I'll post some pictures. To germinate the seeds I grew last year I just planted them and tied them up in a carrier bag to propagate them, I'm going to do the same this year so fingers crossed it will work again.