There are Wi-Fi controlled dog crate locks used for exactly this purpose. The gadget is just a sliding bolt with an actuator, designed to be added to an existing dog crate. They’re not that expensive. (I’m in a wheelchair and have a service dog, so I actually looked into several of these kinds of gadgets last year.)
Once you look at these, you’ll be able to see that they could easily be adapted to any smartthings-controlled plug in power switch. (Some might take a little rewiring. If you don’t want to do that yourself, you could have an electrician do it really easily. But if they’re designed to just plug into a WeMo plug, you just use a SmartThings controllable plug instead.)
There’s one brand, Easyout, which can be used with a plug. That should be smart things controllable if you just use a ST plug instead.
The Easyout inventor’s website looks a little hokey, but the device has been on the market for a couple years and a lot of small pet supply retailers carry it. Take a look at the guy’s videos, he comes across as much more professional there and you can really see what the device is.
http://easyoutpetrelease.com/?page_id=621
And this video with the inventor shows the parts and how it actually works.
One community member, @johnr , made a much more durable weatherproofed device to automatically close a chicken coop door, but I think that would be more than you need for your use case. You can see the basic concept is pretty similar to the dog crate one above, but in a much more heavy duty form.