Nikon 300mm f2.8 vs f4 (sharpness & filter holder capability)

I want to use the lens for landscape (not sports or wildlife)...  The 300mm f4 does allow screw on filters/holders, but I am a bit worried about the sharpness between the two lenses.  I will almost always be on a tripod and will hardly ever be at f2.8...  Also, I would most likely be using teleconverters in some circumstances, so autofocus is not an issue as I will be mostly in manual.

I know the f2.8 lens does not allow for filters or filter holders to be scewed on the front...  However, I was wondering if anyone knows of anyway to rig a filter holder on the front to be able to use square filters?

I guess I want to be able to use filters on the front of the f2.8 as I know if should be sharper with and without teleconverters...

Anyone with any suggestions is much appreciated.

Thanks you

Mark