Other transparent creatures usually have a racial bonus on Hide checks. The static DC 20 camouflage is usually reserved for creatures that resemble inanimate objects. I'll see if I can find a precedent or two to determine the amount of the bonus.
Edit: I stand corrected. The g-cube has this...
Transparent (Ex): Gelatinous cubes are hard to see, even under ideal conditions, and it takes a DC 15 Spot check to notice one. Creatures who fail to notice a cube and walk into it are automatically engulfed.
In that case, I'm fine with simply borrowing that ability.