I have this problem too!

First of all, I'd love to leave this out because my cats ADORE it. They lie in it and use it as a hidey-cave when they're playing, hiding inside and pouncing as the other goes by. However, it gets knocked over REALLY easy. My boyfriend has become semi-obsessed with fixing it and has tried using string, shoelaces, etc. and just can't get them to work. I might try packing tape but I told him "no" for now. The zip ties might be a good idea if I can get a hold of them - but then again, I can see them slipping out as well. It's a matter of the joints just not "clicking" well enough.