If his calibration is right then three perimeters and three top/bottom layers should produce a print with no sagging. I have printed many hollow dice.

This is a simple bridging test and any printer with proper cal should be able to bridge 50mm easily.

I think you are correct. I did not think too much about the size. I always trying avoid to design parts that depend on bridging because the concerns about the strength. However in this case, the strength of the dice is not a concern and the overall size make it unnecessary to use support.

I think rotate 45 degree and use support "touch build plate" should work, may need brim.

If his calibration is right then three perimeters and three top/bottom layers should produce a print with no sagging. I have printed many hollow dice.

This is a simple bridging test and any printer with proper cal should be able to bridge 50mm easily.

Edit :

Voila !

25mm cube printed with 3 top layers, no supports.

Also how do you selective dissolve supporting material without dual extruder?

I assume he need to get support material out.

HIPS can be dissolved  with Limonene which does not harm the other plastic. Since he wants the holes on the dice open the Limonene can get in and dissolve the HIPS then be removed as well.