Tunnel cloches

I've decided to try and warm my wet cold clay soil up with some tunnel cloches this year to extend my growing season. I've got some pipe etc on order but not sure what to use as cover, e.g clear plastic, fleece, thermacrop etc. And whether or not some covers may be of more benefit at either end of the season etc or could have multiple uses to avoid extra costs. Any thoughts appreciated. I may also be trying to use some to get an earlier (and maybe later) crop of potatoes outside as I don't have a greenhouse.