I think the F3x will be too small for a 300 f/4; at least my Canon 300 f/4 IS is bigger than I'd carry in my F3x.You might look at the F2 (where you'd need to get a 3-compartment FA-230 (720-230) insert since the 4-compartment one it comes with won't be suitable for a long lens like most 300s). The lens (with or without camera) would then go on its side. Another option would be (assuming ungripped camera bodies) the F-802 satchel with the 720-230 insert. This is deep enough to accept my 300 f/4 vertically, although not when the camera is attached.
To the best of my knowledge, all the nylon Domke bags are either J-series or have a "B" (for ballistic nylon) after the model name (F3XB, for instance). They also only come in black (the canvas colour options include black, as well as blue, sand, olive and brown). Any bag listed under the "Classic Camera Bags" lilnk [url=http://www.tiffen.com/products.html?tablename=domke]here[/url] will be canvas; any bag under the "Ballistic Camera Bags" link will be nylon.