gehajake A (colony) or box trap as I call it is different from a live trap that has spring assisted doors that close when the animal steps on a pressure plate. The box traps I am referring to have doors on hinges on both ends without spring and very little resistance to the doors. The doors are set at an angle to allow easy entrance but make it impossible for them to exit and remain in the closed position until something swims through them. Once they pass through the door it shuts behind them (gravity) leaving them trapped. With doors on both ends of the trap, setting in a den entrance means you are catching both muskrats trying to enter and exit the den.

You are correct they can have multiple entrances and even multiple dens. Growing up on our 0.6 acre pond the muskrats had 4 or 5 different dens with each having multiple entrances. We would make the trap ourselves out of odds and ends materials laying around PVC, corrugated pipe, metal barn roofing, or steel hardware cloth. A quick search on Youtube and you can find all kinds of ways to make them in a matter of minutes with little effort.