Dinosaur party,
Have had a problem as you, not with a Cage, but similar. There are two points to address, when you’re trying to stabilize a “top heavy” object.
First, get the Base on a stable Platform.
IF you’re on a Tile Floor, then that’s probably as good as you can get. But, if it's on Carpet, use some of the Plastic Carpet Protectors (4 in this case), the conical spikes penetrate through the softer material, and may not actually touch the hard surface under the Carpet, but reduce the instability/give of it, considerably.
Next, is getting the Base heavier than what’s on top of it.
Hopefully, you can access the interior of the Stand in some way, even if you have to cut a hole in the backside, and cover it up after the procedure.
I have used Sand, Crushed Rock, even Bricks in one instance, to get the Base weighted up well. Any material you use, MUST “settle” to the lower section, that’s important, or you’ll defeat the intent.
The heavier you can make it down there, the better it will be. Just consider what type material is containing the ballast, and put as much weight as reasonable in it. If I had to guess at an amount, I’d venture to say to increase the total weight of the piece, about 25 to 30%, and that should help a lot. If you use sand, be sure to check the Base carefully, and tape up any hole or penetration, preferably from the inside, before you start pouring it in.
Hope this helps.
Have a good Day !
