i did the same thing for my 10G incandescent hood, but the spirals are sometimes too big. Mine is very very snug fit and fortunately, they are cool enough that they don't melt the top of the plastic.
I actually just bought a incandescent hood for my 10G so I could switch to compact flourescents. I am currently running 2 13W, but am looking for a better choice.
The FE-IIS-20W-50K is a 20W that will fit in my hood 2" in diameter, it is a 5000K, it also comes in a 6400K FE-IIS-20W-64K, but is hard to find. I am looking for a cheap place online for them.
walmart is now stocking GE daylight bulbs in stores, mine didn't before
I picked up the 15w (60w equiv) and 20w (75w equiv) spirals to test em out, they both fit, the 20w were a very tight fit and were touching the metal backing.
I have a 60 gallon with a custom hood. right now I have 4 27 watt (100 watt equivalant) compact fluorescent bulbs. I have rood for plenty more. My tank is a 26 inch cube so it is very deep. How many of these would I need to get moderate plant growth.
make sure to get DAYLIGHT not soft white..
Today I was at homedepot and had a look at the lights. Heres the pricing on the DayLight bulbs
---pack of 4- 14w bulbs = $8.50 (one i took)
---one 14w bulb $4.50
---one 19w bulb = $6
---one 27w bulb = $7
---one 42w bulb = $13
They also have 9w bulbs but i didnt bother with pricing