I donīt know how much space is left for a knife in DR PSP since I donīt have it but if you are looking for a tiny knife I would recommend Byrd (by Spyderco) Starling. I carried this knife on my keychain and tested it quite a bit but unfortunately I have lost it not long time ago. I think I will buy the same knife because I really liked it.
Despite its size it has quite good ergonomics and can get the job done. I didnīt observe any blade play. It seemed to be built very well.
However another and probably better option could be Spyderco Ladybug (better steel and various colors - I would go with orange for PSP). But it is more expensive than Starling. Byrd Starling has very good price/performance ratio. (Itīs around 12 USD)

I am not really knife expert and I havenīt tried many knives so maybe there are better options out there. This is just my experience with Byrd Starling.

This is my photo of it:


Edit: Spyderco homepage says: "Discontinued - Closeout Price." But it can still be found in various stores.