The F-5xc fits my Olympus E-3, which has a little larger mirror box. Your D300 should not be a problem. I've also used the bag both in a "lens down" and divided horizontaly for use of the pockets (not on the F-5xz if I remember correctly).
And for acceccory, I could fit a lens and a flash. Sadly Domke does not (yet) offer an insert that fits the bag, so I had to go with a SuperFlex from Billingham. The 9-18 is a little bit tall, check your lens length if you could use the 9-15 insted.
Still hoping for a F-3x waxwear :)