OP Here. Thanks everyone for the extremely helpful insight and recommendations; and my apologies for not responding sooner.
I've been doing a lot of research on bags based upon the comments posted here, and one duffel that was not mentioned but has caught my attention is the
Tumi McGuire, with dimensions of 9" x 21" x 10.5" (1985 volume). My biggest concern is with the 9" height (as the bin is measured at 8"); but the previous duffel I was using was 10" and was able to squeeze into the bin. I really like the large single compartment.
The B&R TUC115 that was suggested by thegasguru is interesting; I wasn't originally looking at a wheeled bag, but I am planning on stopping a luggage store this week to take a look at it. Based on the recommendation of the Tristar + B&R, I'm nervous about the capacity of the bag and I'm not really interested in carrying (and purchasing) two bags.
I was incorrect about the Tristar not being cube friendly; but I've pretty much ruled that bag out. I really don't want a compartmentalized bag, and to be honest I'm not a big fan of the way it looks.
The
Patagonia MLC that oshelef mentioned also looks promising (The bag, not the wheelie) as it is 20.5" x 13.5" x 7", and I'm planning on going to investigate this one in person as well.
I did order a small RedOxx Aviator, for $35 I thought it was worth the price and I need a new compressible duffel to travel with on long trips for souvenirs anyway. I think I want a bag with a should strap though, so once I get it I will look into having hooks added for a strap.
Anyway, I'll let you all know how my in-person investigation goes this week, but thanks again for everyone's help. If there's any further suggestions or something I'm missing, please let me know!