First of all, don't use T-Max. Use a fast old-tech film like Tri-X. Push it a couple of stops (I used to get major grain in TX by exposing at 1600 and double-length developing it in stock D-76). You could also try using paper developer, or a high-power/medium-grain developer like DK-50. Agitate A LOT during development to really bring out the grain (I once made the mistake of developing Minox film by hand, seesawing it in a tray; the continuous agitation produced major grain.) You could also try using higher development temperatures to borderline overdevelop in order to make the grain more pronounced.(posted 9184 days ago)And consider using a smaller negative. Either just enlarge a small part of your 120 negative, or do it in 35mm. And half-frame 35mm or submini formats (16mm, Minox 9.5mm, etc.) are even grainier...
Hope this helps!