I had one like that when I trapped in SC. Put it in those small creeks and blocked off some of the creek to funnel the critters through into the trap. Never caught a beaver, but I just wore out a bunch of otter.
If I remember the trap was 8' long with a 14X14 0pening. But I ran a 3" high section of fencing across the bottom of the opening. And it was placed 4" inside the trap opening. This allowed my doors to have a shorter closing distance. And that allowed me to build a shorter trap.