I don't own any, but those expensive holders from Sinar called "Precision Sheet Film Holders" would probably be ideal for holding direct positive paper because they have a pressure plate similar to the Linhof holders to keep the paper precisely in the film plane with a stated tolerance of +/- 0.03mm. The Sinar design opens like a clamshell making it easier to load than the Linhof, which requires you to slide the sheet in from the narrow end.

Cheers,
Rob M.