Gap filler for plastic?

This may seem like a strange question, but I've been searching everywhere online looking at my options. Here's the story:

I have a plastic dog crate that's 2 pieces, a top and a bottom. When they're together, there's about a 1/2" to 3/4" gap between the two. Well, our dog that we recently adopted from the humane society has separation anxiety, so when he is in the crate while we are gone, he gets excited and lifts his leg and urinates in the crate, right into the gap between the halves.

I tried filling the gap with caulk that bonds to plastic, but it's still pretty flexible, and I'm afraid he might scratch it off.

I'm thinking now if I could find something to apply over the caulk, and kind of flash the 2 halves together and smooth it. Is there a spackle type substance for plastic that would work? It needs to be non-toxic. I imagine I could paint over it after its applied and dried.

Any help would be appreciated.