I think you actually want a switch with a plunger that will accept a snap on cap. Those caps would need to be designed to fit whatever cutouts are put in place, but it would allow for more design in the cap and likely a cleaner installation. So something like this?
Really with something like that you'd probably design a recess in the handle at the button placement to match whatever profile you were designing for the button cap. At the center point of the recess would be a hole to allow the snap for the cap to pop in and pop on the button plunger. Take a look at your mouse or joystick to get an idea of how that might work. The alternative would be just simple holes and a longer button post like what you linked. It'd work, it just wouldn't be as clean (but would require a hell of a lot less complicated part design).
The handle makes sense for production and professional button placement. Having the flats to mount a circuit board behind makes a lot of sense for a mass produced product. That'd be hard to accomplish with the original handle design. I had given that a little bit of thought, but not much. I hadn't considered a circuit board in there to mount the buttons to, but I had thought about a plastic insert that would slip over the posts for the screw assembly of the two handle pieces, with spots along it's front edge to glue the buttons. Sort of a plastic web thing. *shrug*. That might make sense for the DIY printer design, less so for a production version.