Q: how to mask edges of D800 focusing screen?

I want to mask the edges of the D800 focusing screen (I need to mask it to a 4:3 aspect ratio project). I need something tape-like that will apply easily (manuel dexterity is not a problem) but remove without leaving any residue but affix firmly enough that there is no danger the masking strips falling off and down into the mirror box. Any ideas? (Note I know about the 4:5 crop format and dimming the finder edges, that's not what I'm after)

(It's at time like this I miss the old Nikons -- I would have just got a replacement screen and painted the edges and popped it in.) Thanks.