When we first moved here 25 years ago, we built a DIY propagator. It was made of reasonably sturdy wood, with a trestle-type base (cross members for legs) and a flat boxy bit for the propagator, about a foot deep. It could have been built with the trestle bit being folding, but we didn't do that as we had space to leave it up all year. The box part had cable laid in loops about 3-4 inches apart, laid on sand, with more sand on top, and the rheostat lies horizontally in the sand, covered. We drilled a hole through the side of the box part, so the rheostat could be controlled from outside the box. Ours was only for germinating seeds so the box part was not as deep as it could be. The hinged top was covered in a clear Perspex sheet and a plant light was hung above. Better lights are available now which cover a larger area. All the info you need is now available online - for us we had to work the whole thing out for ourselves. Most of the instructions are assuming you wish to grow cannabis, but they are easily appropriated for growing normal veggies and flowers. If we had made the trestle a folding one then we could have stored the sand in a sack, folded the base and emptied the box part, then stood them up against the wall. Other changes we would have made were we making it now, would include the use of a bubble wrap or polythene lid extension over hoops to allow plants to stay in there as they grew, and lining the box with reflective foil.
You can make it almost any size you like, but remember that sand is extremely heavy so the support part needs to be correspondingly strong.