Back to the drawing board. After trying to make a mold out of the model, I could not get the model out of the mold, aside from not having enough molding rubber to cover the entire thing completely, which already had cost $50 for 1kg. $50 down the drain 'so to speak'.
So I've built a brand new one already, which more closely resembles the picture with slanted sides. It is also slightly larger at 220mm square at the bottom.
It will not have a removable top, this was going to be too complex for the mold making process, but the model has an open bottom anyway, so it just lifts up.
There is a place up the road from me who make molds of things, but cost could be $$$.
Only option I see left is to hand make every single one out of polystyrene, finished with plaster or grout and sealed.
I would need to know from those interested how much you would pay for one so I can work out if it's worth mass producing.