Slate blue and tan would look really good in a disappearing nine patch with a narrow dark brown sashing...kind of a plaid look. Masculine but also could be contemporary depending on the fabric choices. I would tend to stay with prints that would read as a color rather than seeing a design to the fabric. NOTHING flowery or lacy in any way.
Don't know the name of the pattern but if you are wanting the name to locate piecing instructions possibly this will help. Since it was made from a jelly roll (I think they are 2 1/2 inch strips?) it was strip pieced then cut into equilateral triangles across the stitched strips. Rotate the triangles and stitch 4 together to form a square. I am sure someone has compiled a jelly roll pattern book that will give the measurements or if you are not as mathematically challenged as I am, you could calculate the size to cut the triangles. Hope this is helpful....
Do a Google search for "wild iris patterns". I am sure they have a free pattern that can be printed to any size you specify. You put in the dimensions and it spits out the pattern in that size. Also gives several pocket options as well.
The sagginess is due partly to the position of the handles. The weight in the middle of the bag is not supported by anything and the handle attached on each end pulls the ends up allowing the middle to sag. If there were handles attached to each side like a tote bag there would be support more to the center of the bag and it would not sag as much. Just an observation from my own experience.
Another option would be to insert a stiffener in the botton of the bag...a piece of fabric wrapped heavy cardboard, Masonite, plastic canvas, etc... would lend more structure to the bottom of the bag. If the bottom of the bag can't bend the handle will not pull the ends up.
Here is a Mini-Tutorial on a tote that zips into it's own pouch. Almost like you are looking for but the pouch is on the side instead of on the bottom. Maybe the construction information could be modified to put the zipper/pouch on the bottom.
You might try rivets. Lots of purse straps are attached to the D rings with rivets. I have not used them but I am sure if you googled you could find out how. Your drawing just seems to suggest the addition of some metal unless it is already on the belt. Rivets might also lend a bit more masculinity to the bag as well if you think it might need beefing up.
An addition to the previous post idea might be to stitch the belt on with leather lacing either by running stitch or whip stitch. Of course you would need to use an awl to pierce both the leather and the fabric to allow the lacing to pass through.
Your drawing looks very interesting so I look forward to seeing photos of your finished product.