on third thought, i should be able to get what i want in there.

i have about 5 mms from the base of the fixture to the socket for the bulb. but, then i have another 5 mm to play with to get to the center of the socket. so, anything with about ten cm from the socket should fit - and i did see some of these

i still have a $25 credit at amazon, so that's probably the best way to approach this.