and here's a link to a handmade inexpensive solar water heater!
This greenhouse cost about $600 to complete, including the foundation, electrical modifications and drip system.
If you have fewer wild animals in your area, then you may not need your garden enclosed, but in the desert, I do.
I started in the middle of my circular driveway, an area where our construction trailer was located, so it had electrical and water connections. I used sun-resistant electical conduit, not PVC pipe. The foundation is CMU block filled with concrete and with 3' rebar pounded through the block into the ground, then also protruding upward about 8". I pushed the conduit over the stubs of rebar, then bent them to the other side. The basic frame is an octagon, so I could use 45 degree angle connectors to shape the circle near the top. A rectangle would be easier. I covered the lower dome with shade cloth. Then, the top with bird cloth. The conduit is coated with asphalt emulsion and paint, so it won't break down in the sun. I covered the entire bottom of the greenhouse with chicken wire, so rabbits can't tunnel into it. The shade cloth protects from some sun and the wind. Veggies need so much sun that I couldn't cover the top with cloth.

This is a closeup of the interior with veggies and flowers. I built raised beds with lots of compost and horse manure. The desert soil in this area is very rocky and hard. I added four posts that you see in this photo, anchored in concrete, to help support this in storms. I also put in angled braces and cut little wind holes in the screen. I don't try to keep out bugs, just rabbits and birds. So far, the plants have loved it. I had to surround it with an electric fence to keep the javalina (desert wild pigs) from bashing at it and eating all my compost.