I've never built one . It won't stand on it's own ? I usually see them in dioramas so I suppose they're anchored to the base ,
looking online they all seem to be standing upright unless they're mounted to a base . It doesn't look like the feet have anywhere near enough volume to add any ballast but you could probably get some neodymium disc-magnets into them ; maybe --- would only help atop a ferromagnetic surface of course .
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.
Grue; You are going to need to mount the model to the base (screw/bolt/pin/etc,). The design is not stable. I have one that I have been working on for ages (the old MPC snap together one - photos forthcoming) and they are the same too. And I'll be doing the same thing and mounting it to the base using a bolt embedded into one of the feet.