Keeping Dog From Going Under Trailer?

Our dog got a puncture wound and a 2 in. long slit in his skin from something in the back yard. This is a fenced in back yard. We've checked the fence for protruding nails, the swing set for protruding screws and all that's left is a trailer with some brush on it. Possibly he went under the trailer and snagged himself on the metal.

The vet doesn't have any ideas either from the shape of the wound. A friend suggested a wild animal attack, but that would mean a squirrel or a bird did it, and that's not possible given the severity nor the type of the injuries. So, what do you suggest that we put under the trailer (which is just an open wagon) to keep him from crawling underneath or near it?

Vet said he's going to be ok - drain out in 2 days and stitches out in 10. Poor fellow.