I "inherited" (well, for $30) a big canvas tarp from the prior owner of my H23. It is about 30 by maybe 14. I built a frame from mainly PVC and some wood parts, cost about $100 - I wanted to make sure the sides were about 45 degrees to avoid snow damage, and I wanted to keep the weight off the stanchions and lifelines. The frame has about 7 or 8 A-frame type frame units (with vertical pipes at the bottom to clear the lifelines) and a ridge pole down the middle. It was a pain to construct, but worked well last winter.
If you can find a heavy canvas tarp somewhere, may work for you. The plastic tarps may also work, but I have read they tend to get shredded in the wind - but even if so, maybe buying new ones every few years is OK, as places like Harbor Freight often have them cheap.