Hmmm... I don't understand the notion that the ground is the source of heat and the fabric can't work without it. Solar heat warms the air in the greenhouse and the soil or other media your seedlings are growing in, whether they're on a bench or on the ground. Row cover keeps the warmth from escaping at night.. We use it over seedlings on benches at the farm as well as on the ground. I saw no sign it wasn't working when they were raised off the ground.
One thing I do in my own greenhouse is to use water buckets (tightly sealed) for heat mass. I also have used a couple of planks laid over several sealed 5 gallon buckets as a bench for the seedlings. So that is one way to add heat storage mass and minimize heat loss under cover.
Why the seedlings are up off the ground, I have checked it with my thermometer and it gets a lot colder on the ground at night than it does up on a bench. Cold air sinks. And in spring, there's really no heat left in the storage mass of the floor.. nor much in the earth if it comes to that.
Here's a link to the selection at Harris Seeds - I notice they have a 4 oz and a couple of others on sale at the moment - also they have a good variety of shapes and sizes. I wouldn't mind getting a wider piece for my "top" instead of joining 2 pieces.
https://www.harrisseeds.com/collections/row-covers
FourOaks, there are good prices on Dupont row covers at Walker Plants in NC. They're in your area and would know your conditions well.
https://www.walkerplants.com/accesso...row-covers.htm