If you don't want to shift (you can only shift on the 260 about 5mm anyway), then you can put a 95mm slim on the CF and will not vignette. The problem will be finding a slim 95mm ND, the only ones I know of are the Hi-tech Firecrest line and I don't believe they are shipping either a 95mm or 105mm even though they are listed. I have had the 105mm on order 4 months now.
The Lee system will not fit or work well on the 28mm with CF installed. Lee doesn't make a 95mm wide angle ring. So if you use the standard ring, then you will vignette. This is in regards to the Lee standard 4 x 4 system. The Lee SW-150 only has Resin ND filters, they now offer the little stopper in the 150mm size, but that may be too much filter.
You can use the Lee 95mm to 105mm adapter ring to fit over the CF and hold the Lee Wide angle hood, with one slot.
My setup for the 28mm is the Lee wide angle hood with 1 slot, and the Lee 105mm filter ring on the front of the hood for a CL-PL, Lee now makes a slim 105mm CL-PL, (it's a bit warm) and this will work well with the 28mm. Thus you can have a Lee glass 0.9 or Halida 1.2ND in the slot and use the CL-PL on the hood. You can get a about 5mm of shift before the corners of the Cl-PL show up.
I prefer the hood solution since the 28mm is prone to flare anyway.
Paul