The best portable target I've ever made was a simple 'tripod'.
I used galvanized steel 'strap' like what is used on the top of chain link fence. I have no idea what the proper terminology is for this type of strap.
Anyways, I bent 2 pieces like shown below, used one straight piece, drilled a hole through the 3 pieces, and used a nut & bolt to hole them together.
I then welded a steel plate to the 2 bent pieces. Now this will create a tripod. The center leg is allowed to pivot backwards when you want to set up your target.
This made for an amazingly stable target! It stores easily too.
I made the mistake of shooting it with a SLUG! Gotta build a new one