I don't use plastic at all for frost protection, but we do solarize our beds in the summer, and use 6mm. plastic for that. You can usually get it at Home Depot or Lowe's. The cheap 2 or 3mm stuff only lasts one season, and doesn't resist birds, hail or wind. By the end of 6 weeks of FL sun, it's all in pieces, even when we use 2 layers.
As Anne said, if you take it in before the sun gets too intense it should last you for several years. We get about 2 years out of cheap 6mm. because it is used in the max summer sun for about 6 weeks to 'bake' the soil and get rid of nematodes.
I'm about to buy some for my local school garden, and found this link with 6ft. width for .79cents per linear food. (a 50ft. length would cost $40) but this is 6mm. UV stabilized, 92% light transmission and supposed to last 4 years. (that's probably year 'round use, not temporary use like what we're talking about) I'd think this product would last really well for a tunnel for a couple of months in early spring.
Sewing it to the masonry ladders is a great idea, Anne. You really do need to attach it somehow, it will flap its way out from under just about any weight you put on top of it. Also if it's flapping at all, it's letting in the cold air!
