Have you seen or are you considering something like this? They actually work pretty well, with the caveat that you need to be prepared to open them up on warm days so they don’t *cook* your seedlings. The biggest problem with them is that here in Billings (or for any other northern climate), we are setting them up in early Spring on frozen ground. Thus the cold is trapped inside and temps can get dangerously low/freeze on cold Spring nights.
An option might be to first lay out heavy black plastic on the ground slightly bigger than the footprint of the greenhouse. Stake it down, then erect the greenhouse over it. The sun will quickly heat and thaw the covered ground through the absorbant black plastic – you are also safe from mud because of the plastic barrier – and you get some radiant heating action.