See attached pics.
You're missing a bearing on each side and a bolt through the middle.
The bearings just sit in there loose, they aren't pressed or anything.
Basically, the bolt goes through a washer, a bearing, then the shifter itself (not sure if it has any kind of sleeve inside there or not), then the other bearing, a washer, and it's all held together by a screw threaded into the end of the bolt.
If you can find bearings that size cheap, the rest could be pieced together from a hardware store.
Since this is for home use, you might even get by with a stack of washers with a sleeve bearing inside (all available cheap at lowes).
The head of that screw doesn't have much clearance, so you probably couldn't get away with just using a nut unless it was low profile.
Hope this helps.