F2: add a single compartment insert?

I just bought an F-2 (sand) and so far love it. I'd like to get a single compartment insert to "fill out" the rest of the main compartment. If I move the 4 compartment insert all the way to one end, which insert to I need to get to fill in the rest of the F2?

On an unrelated note, I'm thinking about sewing on some "lash tabs" to the bottom of the back so I can strap on a tripod. Has anyone here tried that? Any tips/suggestions?

Thanks in advance.