While it's not immediately apparent, props are extremely important to Riverworld. The producers must go for accuracy. Thanks to Farmer's excellent description, the dimensions and specifications of the few props are as specific as those for the Old Testament's Ark of the Covenant. Alliance Atlantis Entertainment has no excuse for not insuring that the props used in the series are consistent with the source material.
This is the most important prop. Since every being on the Riverworld depends on his grail as a food supply, each character's grail we be constantly visible. It resembles a smooth, metallic cylinder with a hinged lid and a plastic strap. The bottom of the grail houses dietary information for the specified user, so the converted energy creates food that is consistent with the user's native diet. Occasionally, however, the grails will fill with food that is completely alien to the user's normal diet, like an Australian Aborigine's breakfast made of cow's brains and meal worms.
How Could They Screw It Up?
The most obvious potential error is doing away with the grail completely. I don't know what they would replace it with, because the grail is extremely symbolic and is a constant plot point. Grail slavers, for example, are an excellent source for evil antagonists.
Five feet high, with a diameter of fifty feet, it appears to be made out of red-flecked granite. Three times a day, it flares with roaring electrical energy, deafening and startling even to those who have experienced it every day of their lives on the River. If a grail is placed on one of the 700 depressions, it will be unharmed by the flames and convert the raw energy into edible food.
They also appear to be connected to each other by an underground circuit, which is summarily connected to a power source deep inside the planet. The grail stones also seem to be somehow linked to the process of translation.
There is little else to say about this specific prop; it's quite obvious that the grail stone does not need to be changed.
How could they screw it up?
It wouldn't be easy for Alliance Atlantis to mess the grailstones up. Perhaps they could change the look, or change the structure to something more manageable for a television series.