A simple and effective solution for a tight fitting case would be to get some close nap indoor/outdoor carpeting, cut it to size and glue it in using 3M Super 77. I have typically purchased carpet like this at Home Depot. It is quite thin, is generally gray and has a linear pattern to it. The carpet is maybe a 1/4" thick so there should be plenty of room in just about any case for it. The Super 77 comes in a spray can and is really easy to use. It works like a champ.
If you have the room inside the case then I would suggest 1/2" charcoal foam. It will eventually tear up over time if there is no fabric covering over it, but it will do a great job of protecting your drums. Super 77 works just as well on the foam. Utilize a straight edge and an electric carving knife to cut the foam or pull out the table saw with a really fine blade.
Hope this helps.