The PSP screen films are also 4.3" in size , but as generally the PSPs do not come into contact with stylus jabs etc, I would expect some of them to be cheaper than screen protectors made especially for Archos 605 units.
I would say however that you probably get what you pay for, so if you are buying a cheap ass screen protector, be wary that the protector may become hazy at areas you may tap frequently, and you may have problems removing it easily.
Me, i'm looking around now for a good quality 605 screen protector, as I figure we have a month or two before we get our units.
I wonder why Pandora team doesn't include one free, as everyone who buys one will be in the same boat, and receiving a brand new screened unit.
Surely the best time to put on a screen protector is when its brand spanking new ?
( Perhaps they could consider it a small gesture of goodwill for our patience ? ^^)
So, in answer to the original question, the best screen protector should be a free one