I built a Hot bed many years ago. Pretty basic, and very cheap. I dug down about 2 feet, but in some very fresh horse poop, sifted the top soil back in over that, took 4 bales of straw, 2 in the back, one on either side. Had it facing south and put some old single pane storm windows I had over the opening at about 45 degree angle. It worked GREAT. All of the seed germinated, plants were growing well, getting big. I had to crack the glass open everyday so that it did not over heat. One day I was late for work and did not open the glass, Cooked every thing in the Hot Bed. I was working then, now that I am retired and have more time maybe I should try it again. MMMMMMMMMMMM,,,,Maybe I should just buy a couple of plants,
Never doubt that a small group of dedicated people can change the world, indeed it is the only thing that ever has.