It has been years since I used row covers for early set out plants. A generous friend from New Jersey sent us a set of three frames he makes to support a sheet of corrugated plastic roofing material. The camera makes it look blue, but it lets in most of the light. I used baling twine tied to one side of each support, and across the top to hold the plastic in shape. The other end of the twine goes under the cement blocks to hold it down. It is easy to move the blocks for access.