Maybe set a model. Make it almost completely invisible with set_user_rendering(box, kRenderFxPulseSlow, 0, 0, 0, kRenderGlow, 1)
And you could use milkshape 3d trial and make a 10 x 10 x 10 box.
Btw. That's 1 dm3 = 10 dl water

I think that should do it... Or set a sprite