Use a lower quality "chocolate" like a Callebaut d8-11, something with around 45-50% cocoa content. Once you freeze chocolate, the flavour doesn't really come through so there's no sense in using the good stuff for this purpose. What people want is a thin coating they can "crunch" through. If the coating is very thick, it isn't very fun to eat.
DO NOT USE "baking chips" or the like. Remember, if these things won't melt in a 1/4" thick cookie in a 350 F oven for 20 minutes, they won't melt in a pot either.
Thin out the chocolate with the veg oil. Temperature is not really critical, but the hotter your chocolate is, the longer it will take to "dry" on the frozen item, and the softer the frozen item will be once the chocolate hardens. Anywhere from 32-36 C is ideal. Soup warmers don't really have a "low" setting" for this range (it is right in the "danger zone" for perishables), but a crock pot or heating blanket will work.
Hope this helps