Here is a cheap option (kinda ghetto/shade tree mechanic style), but at worst you'll end up having to replace the resevoir tank anyways:
http://www.renntech.org/forums/index.php?showtopic=1838
"hey update on my experimental fix.... it looked like it worked! The JB weld was actually able to stop the coolant leak. After i wiped off the residue and cleaned the outside of the tank. I just put globs of that stuff underneath the coolant tank. It completely stopped the leak, seems that stuff can handle temperatures up to 500 degrees BTW....i drove for a few days already with my ghetto fix, and its holding up well "
Good luck with whatever you end up doing,
Larez2