In buying your “engineer bags,” is it worth paying extra to get one with a flat bottom, or is round fine? It seems like round would roll off you when you try to do a weighted pushup? On the same note, it seems like you might be doing a disservice by continuing to call it an “engineer bag” when searching for that turns up only different things, while “duffle bag” is mostly correct. As a reader I’d rather just see links to a few inexpensive “recommended” ones on your site, though.
I apologize for the confusion; for effectively all of my life I’ve heard them referred to as engineer bags. It’s a heavy duty canvas bag that’s typically early 20th century issue, available at pretty much any Army/Navy store, suitable for carrying tools or parts in, and thick enough that chunks of metal won’t slice through it. “Duffel bag” to me is the big canvas tube-like backpack you see infantrymen carrying their clothing and worldly possessions in. And most people seem to think of the thin nylon/plastic ‘gym bag’ as a ‘duffel bag’ as well.
Whenever I’m not at home and I’m building a new engineer bag (which happens frequently if I’m traveling, at a workshop, helping a friend/client put one together, whatever), it’s pretty much an hour job. I go to a local Army/Navy (or failing that, sporting goods store) so I can physically touch the bags and see what seems most durable (and grab a bag of tube socks while I’m there), then go to a hardware/home improvement store and get 2-4 bags (100-200lbs) of gravel. About $20USD total cost. I usually take the recipient of the bag along so they can pick something that appeals to them (some people don’t appreciate having an olive drab sack in the corner of their nice living room).
I would feel uncomfortable recommending ones off Amazon that I haven’t tried, so I’ve ordered a couple and will test them out and let you know what I would recommend.
As far as flat bottom or round, my current pair are rounded. The rocks will ‘flatten out’ on the bottom and be more of a significant factor in stability than a seam will, for push-ups or whatever. But it’s not that big a deal either way.tl;dr get something durable that pleases you aesthetically. Straps going all the way around the bag for support. Many readers also recommend handles on the side as well as the top.