The "secrets" (which are clearly stated in the instruction sheet, but I figured them out by trial and error before locating an instruction sheet, which I've since posted a few times on APUG) to easy loading are:
1. Testing with a scrap sheet of film on four sides of the cage, set the top plate about 1/16" larger than the width of the sheet.
2. Load the sheets emulsion side in.
3. Bend the sheet to follow the curve of the slot as you load, and it should slip right in.
I haven't tried interchanging caps, but the cap that some have said is interchangeable with other Nikor caps from the same era (I haven't checked this myself) is the small cap on top of the tank lid. The tank itself is the same diameter as Nikor 220 tanks that use the large diameter 220 reels with the same heavy wire as a 120 reel (there are also 220 reels that use thin wire like a 35mm reel, and that fit in a conventional 35mm/120 tank), which is larger than a standard 35mm/120 tank.