You don't have to use the cases if you don't have any.. The AA battery is the same length so here is something that should work...at your local hardware or home center you can find different sizes and width's of foam weather stripping with one side sticky. Just find the right thickness to place under the battery to make up the difference from an AA to C and you should be good to go.. Its been so long since I bought my cases I will have to check where I got them although I know it was Ebay and from China or somewhere. Will check.
Here is an example although not the same seller I bought from. Very cheap. $2.12 for eight of them and free shipping. There are many available from different sellers on Ebay so check them all out. Conversion Case and Many More Here
Here is an example although not the same seller I bought from. Very cheap. $2.12 for eight of them and free shipping. There are many available from different sellers on Ebay so check them all out. Conversion Case and Many More Here