What weight leather works for a duffle without feeling floppy?

For a relaxed round duffle, 5-6 oz for the body is where I'd land. I built one a couple years back and started with 4 oz panels that slumped like wet noodles the second anything went inside. Went up to 5-6 oz veg-tan and it holds a soft shape on its own but still slouches naturally when you set it down empty, wich is the look you're describing. Your 3-4 oz will work fine for gussets and end panels where you need it to wrap around, but the main body panels will fold and crease at that weight, especially once he's cramming it into an overhead bin. If you want a little more structure without going boxy, you can skive and fold the top edge over a light strip, but honestly 5-6 oz alone does the job. Handles are where I'd definitly overbuild. I double them, so cut two layers at 8-9 oz total or laminate two 4-5 oz pieces together, then skive the ends thin where they stitch into the bag so there's no bulky lump. The failure point isn't usally the handle itself, it's the attachment. So run the handles into the seam or under a reinforcement tab that's stitched through the body panel with a decent stitch count, not just surface stitched to the outside. Some people back the attachment area with a scrap piece on the inside too, which spreads the load and keeps the leather from stretching out over time. One more thing since he'll be hauling it through airports: don't cheap out on the zipper. That's the component that takes the most flexing and abuse on a bag this size, and a busted zipper on a leather duffle is basicly a full teardown to fix. Buy the good one the first time. Also, if you've got extra of that 3-4 oz, cut a mock panel and fold it, roll it, and load it with some weight before you commit. Ten minutes of testing saved me from a very expensive mistake on mine.

One thing I'd add on the stitching side, don't skimp on thread weight where the handles land. On the duffle I finished last spring I used 277 bonded nylon through the whole handle attachment area and dropped to 207 elsewhere on the bag. It looks chunky in person but that's the spot where you want the visual weight anyway.

Also agree the 3-4 oz is fine for the end gussets. I used a lighter weight on my ends and they actually wrap and shape nicer than the heavy body panels did. Is he planning on checking this thing or carrying it on? That changed my layout a fair bit, I ended up adding a second interior pocket after a couple of trips.

One thing I'd plan for early is the shoulder strap if he's traveling. On the weekender I made, I stitched the strap anchors straight to the body and they started pulling at the leather after a few trips. Rebuilt them as tabs that wrap through the end seam with copper rivets backing the stitches and they've held up ever since. Also throw a scrap of the heavier stuff under the bottom panel if you have any, that's the first place mine showed wear from being set down on every surface imaginable.