Everything I have read states that it is NOT a good idea to use a pelt.
Reason has to do with the fact that you are dealing with a mechanical device that is suseptable to thermal stresses on both ends of the spectrum. If you chill a moving device too much, you could cause the metal in the device to constrict (on a microscopic level) and cause extra stress on the moving parts.
Simply put, dont use a pelt
Fans work well and watercooling would work even better. HDDs are not like silicon chips, they dont create a lot of heat really quickly. The heat tends to build up over time to the point where it starts to affect the moving parts. A water cooling device that has a low flow rate should easily be able to keep a HDDs temperature level under control with a very low flow rate.
Thats my advise.