You'll run into problems with brass. Modern shotguns are sized to paper or plastic cases, so the inside diameter corresponds roughly to the bore diameter. A brass case is thinner, so the inside diameter of the case is larger. Your slug will rattle around in the brass case. On the other hand, if you size your slug to fit inside the brass case it will most likely be way over bore size.

Inexpensive brass cases (Magtech) are available from Huntington (12 ga. only), made for large pistol primers. They're only about $16.00 for boxes of 25. Huntington also offers a 12ga. shellholder for standard presses (about $20).

Curl