Advice modeling simple block sliding on floor

I'm getting started with dm_control and am trying to model a bare bones environment: a box sliding around a floor.
To do this, I adapted from [mujoco-worldgen](https://github.com/openai/mujoco-worldgen) just a box with 3 slide joints and corresponding actuators. I also use the floor arena and then edited down locomotion walker/target tasks to be as simple as possible.
XML here. Note that if I don't include joint armature = 1, then it immediately fails w/ `Nan, Inf or huge value in QACC at DOF 5` at time t=0.000. When I include it though, the box does not move.
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Full zip of code is attached.

[ice 2.zip](https://github.com/deepmind/dm_control/files/9504813/ice.2.zip)