Conid Kingsize Bulkfiller. Large (MB 149 size), though not padded “section” (that’s the grip), shit off valve so there’s no way the ink can leak with movement or pressure changes. It comes in demonstrator (the name for clear barrels where you can see the mechanism and ink) or ebonite - a black glossy hard rubber. They are the best pens ever made.
https://www.conidpen.com/kingsize/
If you’re serious, head over to the Fountain Pen Network - they’ll have much better advice than on here.