I don't know what size ferret you have, but I have a fairly petite little guy and try as he might he cannot get himself through the grids. I built him a 2 story cage using the regular grids and it hasn't failed to contain him so far and I promise he has tried with all his might. LOL Just be sure that you use zip ties to hold everything firmly in place, especially at the corners, where at least my ferret prefers to try and force himself an escape.
If you just don't trust the grids with your ferret, you can always double grid the cage as is shown in the baby proof section of the photo galleries.
The solid cube grids that my Target sells are actually poly rather than mesh.... I'm afraid a ferret would scratch right through poly material...not to mention it would offer next to no ventilation and wouldn't allow the ferret to see out of the cage.