Good idea! there is a good bit of space where the bulb holder goes.
There are still some deals to be found on the fultons on ebay, especially if you are looking for more than one. BTW, most fultons will run on C cells without an adapter, the tail spring is just long enough to make contact, though a couple quarters dropped in before the tail cap is screwed on will add a bit of tension.
Running CR123's is a bit trickier because of the shorter length, smaller diameter and in that the spring may not contact the base of the CR123's without bending it or adding something to make contact, I got around this last one with the 2C to 2D adapter which has a washer on the base to make contact with D cell lights (Fulton, Mag and others) without modifications. The round head screw I used for the positive end of the 18650 to 2C adapter and the nut on the inside allows me to use button top of flat top 18650's.....only problem is there is no reverse polarity protection with this adapter, though I think the Nite Ize drop-ins are designed to work with reversed polarity.