My solar tunnels are above the ground and I made it on some specific way. I used technique of designing mountain or alpine tends, because of very strong wind blowing away house roofs at my specific area. First I tried polyethylene than PVC tubes, but finally I made it from composite armature. It is very light but stronger than steel and made of epoxy and glass, lasting almost forever.
There are tree ways I am heating solar tunnels:
1. underground rows for air circulation
2. kurgan - mound of composting biomass which produce heat
3. heating with electric wires