I wonder if there is a way to rig up a large dog kennel with the kittens in it. (Got friends with dogs?) Maybe use some chicken wire or something to keep them from escaping, while keeping the main door open for mom. Put tuna in too.

This means the door won't shut automatically - so you will have to watch at the time when you think the mom will be around. Maybe put it near the juniper bush?

Sometimes pet or agricultural stores - even wildlife centres or zoos - might have a live trap they can lend. Even Pest Control places.

That's a lot of phoning, but it's all I can think of.

Good luck. The babes sound adorable!