Comstock  traps designed for beaver work equally well for otter and muskrats.  Pictured is a 12x12x36 double door swim through trap.  We are also finding that even the smaller 9x11x36 double door traps will take otter too.  Either of the 12x18x39 or 12x12x36 beaver traps work great for otter and even the smaller less expensive 9x11 trap never designed for otter but primarily used for raccoons is a winner for otter.