Opossums like tight, secure spaces, with small entrances to protect them from larger predators. You can place an old dog carrier crate (found at garage sales or placed at curb for trash on trash days) and remove the front door from it. Set it atop two cinder blocks to elevate it above any standing water after a rainstorm. The cinder blocks can also be utilized to run a bungy cord through their middle and attach to the airholes on the carrier. This will keep wind from blowing the crate over and predators from tipping it over.
Place an old rubber door mat (or purchase a length of rubber matting from Home Depot) atop the carrier to keep water from coming in the air holes. Top the rubber mat with two bricks to anchor it in position atop the carrier (see picture).
Make sure the rubber mat isn’t too long where that it would restrict air flow through the carrier side and back air holes (see picture). It must be long enough to keep rain from coming in (see picture).
The rubber mat must be long enough to drape approx. ¾ down the entrance of the carrier, creating a “tent flap” for privacy, protection and weather proofing (see picture, facing outward for example, it would be kept turned towards the fence).
Place the carrier facing towards a solid object, wall or fence in the corner of the yard. Allow about a 5” clearance to get into the carrier.
Inside the carrier, lay down a couple of layers of flat newspapers in the bottom, then stuff it 3/4 full of crumpled, clean newspaper. I use newspaper because it’s the cheapest sterile material available.
The opossum will rearrange the crumpled newspapers to its best advantage for warmth, privacy and protection and an alarm system and push out what it doesn’t need. It may add twigs, plastic bags, leaves and debris found in the area and actually seal itself in using the material as a barricade/door. Having wadded up newspaper at the front of the den box will alert an opossum to any intruder by making a crinkling noise. Check and replace soiled or damp newspaper every couple of weeks. Opossums make pretty elaborate “nests” for their 2-3 day stay.
You can also go online and find several different ways to make winter stray cat shelters (cats are a similar size as opossums) out of old Igloo coolers, Styrofoam coolers or even Rubbermaid tubs. In making these structures, you want to make sure that any opening you cut is filed down or taped off so as not to cut an animal going in and out. The opening should not be more than 12” in diameter. If an opossum can get its head in, it can get the rest of its body in. Whatever structure you use, make sure it is weighted down or stable enough that an opossum’s busying around, rearranging all the nesting material you’ve placed inside doesn’t knock the structure over with the entrance face-down, trapping the opossum in the den box. You may also consider constructing a lip at the entrance or a smaller diameter entrance.
Having a den box in your yard can provide an excellent opportunity for your children to “go hunting” for wildlife. Provide them with headlamps and search your backyard after dark and see if they can locate wildlife in your foliage or atop the fence. Have them observe the wildlife from a respectable distance, without hampering or hindering the animal’s movements. You might wish to rake the surface in front of the den box, or layer flour or baking soda at the entrance and rake until smooth. Examine the area in the morning for footprints disturbing the material in front of the den box. Take cell phone pictures of the footprints without disturbing a possible sleeping animal inside.