I've had customers attempt to dry out foam cores many different ways mostly without any success, best thing you could do is buy a new cover before someone slams down your current cover and it damages your shell, I've seen covers so heavy they have actually cracked around the 'opening' where it's cut out for the topside controller when they were dropped or slammed close.
I don't sell foam cores either, between what they charge for them + shipping costs it makes 0 sense for the retailer and for the consumer vs just buying a new complete cover.
Good Luck!