Assuming you mean to use the grip shift as a base, you would just need a way to introduce enough friction in the system to hold it in gear against the derailleur spring. If it's an index shifter, it would already have the indexing in place and, if you didn't want it to index, that's an easy change, as well.
Here's what I made for
Retro Rocket
The shifters are attached to a piece of handlebar laterally across the rocket body (originally, the wires for the light switches at the ends of the shifters were supposed to go through the center of it, but that wouldn't work long term). They basically work as normal, I just used a shaft collar on the rotating piece of the shifter and drilled it for the shifter tube to pass through....
To secure the rotating piece of the shifter which is normally held in place by a grip, I made this doohicky (below) to sit inside the "handlebar" tube:
The shifter tube passes through the eye bolt (actually a modified thumbscrew) and that's a bearing it's bolted to that allows the shifter to rotate while the tail piece is fixed to a short slot inside the "handlebar" tube with a bolt and that threaded panel clip above (you can see the slots—like elongated holes—in the middle pic, but this doohicky wasn't installed at that point). It could have been bolted to a regular hole, but due to the increased leverage of the shifter over the original design as well as a reduction in friction within the shifter body due to the missing grip and removing a large chunk of the fixed part of the shifter to allow for the shift tube, I also wanted to be able to put some tension on the shifter, so I elongated the holes to allow for some tensioning where the eye bolt pulls the shift tube inwards. If I end up remaking this rocket body in fiberglass, I'll redesign a better tensioner as this one requires a large C-Clamp to set it and doesn't allow for a lot of play. Works pretty well, though.
