I found this question so interesting that I joined eHam just to continue the discussion
Why not take advantage of the feed line as another radial? I'm thinking of placing my choke just outside my radial field. I also wonder if this might be more effective at keeping RF out of my shack than putting the choke at the base of the vertical. With the choke at the base RF currents might couple back onto the feeder at close in distances where the fields are high. By putting the choke outside the radial field where the fields are lower there will be less opportunity to couple back to the feed line.
With regard to snap-on chokes.. The inherent problem with them is that any gap between the two halves will reduce their effectiveness. It is better to use solid cores and thread them onto the coax before the connector is installed. Fair-Rite makes both snap-on and solids in all the materials.... 43, 61, etc. DigiKey used to carry them - not sure if they still do. The generic EMI suppression snap-ons that some suppliers carry may not be a suitable blend. I'm not sure how many cores would be required to make an effective choke - perhaps a dozen or even more. W2DU published some work on baluns with these cores years ago.
Edit: found information on W2DU's choke http://vk1od.net/balun/W2DU/index.htm
He is recommending 50 cores using type 73 material for HF.