They sell the them around here. Unless I'm misunderstanding what you are talking about, the doors here are pushed open by the hog and then the hog can't find it's way back out; but other hogs can keep pushing their way in. We don't have a spring one but we do have a drop swing and it works very well, usually catch 4-5 in it.
I like this style and Wisdom Mfg out of Jourdanton used to make them. It was very similar to this one and you could buy just the gate portion to build your own size trap. I have seen large groups of hogs caught in this style trap. They set them in the shade around a tree with feeder and waterer. They are continual catch style traps. Like this one GS?