This question is related to a 120V North American situation, but others may make suggestions as well.

I just built an accent table lamp. It uses a 7 watt PL-S 2-pin CFL. Since the fitting is too small to house the ballast, I built an external ballast case out of a metal single-gang switch box and cover. The ballast is mounted on the metal cover, and takes little space inside. The intention is to have the lamp on a table, and the ballast box on the floor beneath the table. I have it running now for about four hours. The ballast box is on the floor, with the cover facing up. The ballast is mounted on the cover, and throws out quite a bit of heat. I cannot comfortably hold my fingers on the cover, directly above the ballast. It seems to me that it is heating too much. I have a different brand ballast running an 8W T5 lamp, it's been running all day (15 + hours) and is not as nearly hot as the PL-S ballast. I don't know the manufacturer of the F8T5, but the PL-S is made by Standard, and is model number 56615. The thermal cut-off has not tripped, but I worry that the plastic twist-on wire connections in the ballast box will suffer.

The question is whether this is normal or if I should add more metal to the box to make a heat sink.

Thank you in advance for your replies ; it is greatly appreciated. It is really nice to be able to ask this question to people who not only knows the answer but who cares and understands the lighting situations one can encounter.

Are they the same size ?

On some ballast the temperature rise more then 50degC above the ambient is normal. But it mean more then 80degC operating temperature, what is, indeed, too much for not "high temperature" rated components (mainly wires, connection material,...).

Preferred (by me) option:
If you have some, use the F8T5 ballast (the type you mention to not heating as much), it is electrically the same, so it will work with the PL-S (all, except 13W) as well.

Other option:
Use higher temperature rated material for the ballast box (and of course it's inner installation; at least 120degC).

Better is to use a grounded metal box, and fix the ballast to the case to provide heatsinking to keep the ballast cooler. A hot ballast is more likely to fail.

Thanks for the help. I will experiment with heatsinking. The ballast is already in a grounded metal box, using the side mounting bracket instead of the edge mounting bracket. It is quite well in contact with the metal box. One electrical cord connected to the ballast is rated at 105 degrees C, but I didn't check the other one. If it is lower than 90 degrees C, I will replace it. I doubt very much that it will ever reach that temperature, but just to be sure...

When I get the chance later on, I will post pictures of my setup.