Bags of insulation on top is what I would recommend - but - if you decide to go down the artificial heating route, then place any heating device 'below' the brood box - but not using that fluorescent lamp !
The reason people have used tungsten-filament lamps for heating in the past, is that they are inefficient, with much of their energy consumption being turned into heat. Fluorescent lamps on the other hand are efficient devices with nearly all of their energy consumption turned into light, and very little into heat - which, on this occasion, is exactly what you want. I'd be very surprised if a 15W fluorescent lamp produces much more than 1 watt of heat. You should be able to hold the barely warm glassware of such a lamp quite comfortably in the hand - thus proving the point.
Far better, imo, is to source a purpose-made heater such as those used for heating terraniums. I have made my own from Aluclad resistors and aluminium sheets (which I've made and tested, but never used), and will advise how to do this if pressed - but for a turn-key solution, think 'terranium heater'.
LJ