You can fashion the frame from very inexpensive pvc pipe available from Home Despot. It's easy to cut with a hack saw, and bends without heat application. The number of bents can match the stanchion spacing and you can use the Ts and Xs etc to join it all without glue. When you bend it the tension prevents it from coming undone.
The pipes and fittings will cost about $25 and take a few hours to assemble. You can lash the pipes to the stanchions and or tape them.
Then you can get a commercial
operation to place the shrink wrap with vents and a zippered door for 3 or $400. Mine withstood a very nasty Nor'easter which I was certain would have blown it all off.
Unless you are planning on covered winter storage
a custom canvas
top and pipe frame is not economical... and you have to store it all in season.
The plastic is recycled and reasonably inexpensive and you can cover just the cockpit
area and not the entire boat as another savings. You don't want ice in your cockpit