Think of some other method to allow the player to bring a cube through. Maybe a small side slot that they can slide the cube through next to the fizzler, and they pick it up on the other side?

NunamedDragon wrote:
Or change the colour of the grill

What they said. In Portal Prelude they changed the colour of the grid from blue to green, with a voice over from some 'employees' that it won't destroy the cube.

NunamedDragon wrote:
That's where i got the idea from...

Woot! Go Portal Prelude.

NunamedDragon wrote:
Or change the colour of the grill

Yeah, that's what I was thinking. Maybe a bright green color or something. I just hope the creator of Portal Prelude doesn't hunt me down :\

I also like the suggestion above of putting a cube-sized slot in the wall.

NunamedDragon wrote:
Or change the colour of the grill

I like the idea of changing the color, and using signs with arrows and all that, but you could make it even more obvious by doing this:
Right after the player gets out of the entrance elevator, before the rest of the test, put a cube dropper in a small room behind glass, so that the player can't get to it. Put a floor button under the cube dropper so that the cube will drop right onto the button from the dropper. But in between the floor button and the dropper you could put one of those green fizzler things that only fizzle your portals. Make it so that the dropper is activated by the player pressing a pillar button using the E key.
So the pliayer will see the button, and press it with E. The cube wll drop through the green fizzler and onto the floor button. That button it drops onto will then open a door, allowing the player to proceed onto the actual test. That way, players will instantly be able to tell what's special about the green fizzler. People usually learn best through demonstration.