John, for letting me measure your man-bag), I did some knitterly math. I'm ready to cast on tonight!
The bag will be knit in 4 pieces. See the little paper model? It has 3 stripes (green, yellow, orange - the yellow and orange kind of blend together in the photo). Each of those will be knit separately, then seamed together. The bag front, back, and flap will be knit as one long piece, then folded like an envelope. I'll include some garter ridges at the folds to help the fabric turn the corner. The fourth and final piece will be the gussets/strap. The gusset on the yellow side of the bag will be green; the gusset on the green side of the bag will be yellow; the strap connecting them will be orange. It will be knit in one long skinny strip.
Bag lady doesn't want her bag to be too heavy, so I am knitting a single strand of worsted weight yarn on a US 10.5 needle. Normally when I felt, I use a double strand of worsted or bulky weight yarn. Not here.