>>1622251Use bottled, "spring water."
>>1622358•Plastic for heat retention. Use 2 layers for very cold places.
•Insect cloth (your pic) for keeping insects off.
•Shade closet (thicker than your pic or woven plastic netting) for adjusting levels of sunlight to the plants.
All 3 can stop general frost.
Mulches like hay will make the ground colder by insulating it from the warm air in the tunnel. Using black plastic sheeting or water-filled black-painted bottles will be best. It will help turn solar energy into thermal energy. The water bottles will help trap that heat and release it at night or during cold snaps. Bottles are nice since you can move them around as needed. The shape of the structure isn't all that important. You can use just about anything so long as wind doesn't take it down and the materials used does not tear the covering.
The supports in that image are black plastic gas pipe. For small tunnels, most people buy 10-feet long, grey, conduit pipe (pic). Those are perfect for small tunnels that don't need walk-in height. Those + rebar inside them are good for larger tunnels when put together to make a 20-feet long pipe. These structures are temporary and easily moved and stored. Whenever you use 2 layers of plastic, make sure there's at least a small air gap between them. Some people make small polytunnels inside walk-in polytunnels. Each layer gives you about a 1 temperature zone warmer micro-climate. So, that Zone 7 will be 8 or 9 for 1-2 layers of plastic.
Another popular structure for winter gardening are cold frames. They are just slanted wooden boxes with a hinged lid on top. The lid is made of glass or plastic. You can also use 2 layers for that. They are good for permanent places, but can also be picked up and moved with some help.
Regardless of what you use, make sure to allow lots of air flow when things are not all that cold or the plants can cook in them.