So you're using a big block of fluval (or equivalent)? I put my usual blue foam in there - only I had to roll it up several times to cover the sheer size of the cart.
Nope, I'm using a filter fiber that's loose in a bag. For all aquariums. 'Loving Pets' brand. I roll it between my hands to make an appropriate density (this is the key) cone-like cylinder of fiber. Inserting the pointy end into the cart, then trimming the excess off level to the opening.
Are you using the silicone cap as a zfm or something else? I tried a plastic paperclip and blue foam but it leaked a lot.
For the Cone (not cylinder) atty, use the silicone cap off a normal 510 cartridge. The one thats flat on two sides, fits exactly into the cone atty cartridge. Just trim it down a bit, cut a vee notch in the flat sides and punch a hole in the middle of it with a metal pen end that's been filed flat.
I'm very disappointed, I REALLY want to use the 2.5ml carts - I want to just grab the 900mah battery, pop a atty&cart on and just go with it for the majority of the day.
These carts will never hold that much, I wish they did! It's almost 2 ml. When I go out, I always carry at least 2 units, usually 3. Sometimes 1300mah Hello 016s or 1000mah Tornados or 750mah Rivas or 650mah Tornados or the GGTB or some combination. I would never just depend on 1 unit. Everything lasts better if you don't over use any one unit and rotate.
I've futzed around a small bit with cartomizers, but didnt like em very much.