I have been also working on a far more stable entrance slit design and decided to include it with the build instructions. I have tested it several times and will post my results, I made one for my 0.12mm slit and my 0.09mm slit.
The slit I am using now is the 0.09mm one.
Supply source for blackout material and the aluminum black tape I used:
http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=190 – blackout materials ect.,
T205-1.0 - AT205 Black Aluminum Foil Tape 1" x 81' (25 mm x 25 m) Roll Cost-$28.80 a roll