How about making one from old windows and doors:
We had thought about doing one like that but I thought we'd get some other options.
By the way, Happy Memorial Day and thanks to all of those who served
The Instructable site has several DYI type greenhouses. Or you can surf the web for various plans. The hoops house types are nice.
I built a simple hoop type rig using short lengths of rebar as "stakes" and then slipped some pcv pipe over those. These ribs were then covered with some poly sheeting..cheap and easy but a bit weak in the long term durability. (The plastic needs to be changed each season). I used this to extend my growing season a few weeks. Being in Texas...you already have a decent growing season.
If I were to build this now I would probably go with corrugated poly-carbonate sheeting.
It is what is used in many of the basic "kits"
We got our plans from the Agricultural Extension Office. Very simple & inexpensive :~)
You should be able to find some on Mother Earth News or Organic Gardening (now Organic Life). Johnnys Selected Seeds has supplies fr simple hoop houses out of PVC and plastic. Or you can buy some pretty cheap and easy to assemble from Hoop Houses.