I use trays from a company called PG Horticulture, they are semi rigid and will last for many seasons. They also have holes in the bottom to push the plants out.
You need to order minimum numbers but it’s well worth it if you can find some people to share the cost (i think the numbers are about 25 for a 77 module tray)
http://www.pghorticulture.co.uk/level2/propquick.html
Hope this is helpful
Rob
