Yeah, it's a bit of an issue with this particular kit.
Attaching it to the base, if it's going into a diorama, is one solution. I started using 2-part epoxy putty for groundwork on my bases. With this model, I'd press the feet into the putty before it sets, to make footprints. When the putty cures, when I'm ready to attach the walker, I'd glue it into those footprints.
I think of two other solutions, involving putty.
One is to fill one of the feet with putty, and embed a small bolt in the putty. Then I'd drill a hole in the base, and attach the model via the bolt. I use fine bolts to secure aircraft with tricycle gear to bases, too.
The other is to use putty to fill both feet, to weight them. I add BBs into the putty to increase the weight. That's another techniqe I've used on tricylce-geared airplanes. It might make the feet heavy enough to keep the model from tipping.
Hope that helps!