There are a number of ways to make an a-frame. What I did for my dog – about 50 lbs, might not work/be safe with a heavier dog – was to take two old wood doors. Attached two piano hinges at the top. Attached one hook on each side of the door (the part that would fit into the frame). Attached a chain between the hooks for easy in lowering/raising. Cut a piece of ABS (could use PVC) to fill the gap at the top. Then I added some strips of wood for a low rise slat, and then sanded and painted it three times. It is not light (in fact is hard to move), but it works just fine for home use. I don’t have to worry about the wind knocking it over either! 🙂
You can find a plan for building one here.
Emry’s Corgis have a plan as well.