Well, here's a jumping off point. This is a 6-slot wide, 90% (38px vs. 42px) bag mod. No title, inset drag handle, inset close box.
** This only partially works!! ** (Don't download this unload you understand that it's not a release version).
It's not sizing properly vertically. I had to set the minimum size vertically to accomodate my 32 slot boxes. Regardless of container size they're all fixed at 6x6.
Thusly, I'm using this as is, but it's a pain because if a container is less than 32 slots I've got MASSIVE dead space.
Maybe someone else can dedicate more time to this. I'm off to work and have no way to test while there. Good luck.