Trying (and failing) to crochet a rug

I’ve started this particular rug over so many times because I can’t seem to find a good way to get it to not warp or bubble. It’s not easy making a rug without the tension a frame offers, but I’m convinced I can do it, I just have to work out a) what I want and b) how to accomplish it.

Each and every one of these pictures is a new attempt:

This is supposed to replace our breaking kitchen rug that sits under the stove. That means it needs to be sturdy and ready to be washed often. But as I keep having to frog the whole thing when it starts warping, I may just have to look into some kind of loom if I want to make it like this.