re: Sound proofing a small space 5 feet wide.

As stated by previous posters, rugs/carpet on the floor is a great option. For the walls, anything made out of foam, such as those "egg crate" panels or mattress toppers, is an excellent inexpensive sound absorber. You'll want as little concrete and sheetrock to be exposed as possible.

Also, consider using smaller diameter/lighter weight sticks (if you're not already). I play restaurant gigs all the time, so volume is a big issue. I've been using 7A Vic Firth Maple sticks for some time now and it's been very helpful in "forcing" me to play softer; those sticks will not last if you're a heavy hitter and, unlike hickory drumsticks which give you plenty of notice before breaking, the maple sticks are fine one second, and completely broken the next. It really encourages a lighter approach.