It all depends on the construction they used to create the viewfinder. A lot of 'BIG' viewfinders are actually just portholes, the smaller the viewfinder the more optics you generally need to get the same field of view.
For 28mm, the easiest way to create a cheap finder is with a Konica C35. These are available really cheaply, and the viewfinder already covers 28mm nicely. There's framelines for 35mm, but I've removed those. A lot of viewfinders on P&S cameras cover more then 35mm if you disregard the framelines, but I've found that most of them are more like 30/31mm, and are still a tad tighter then a real 28mm finder. The C35 finder is the closest cheap option I've found.
Basically the whole rangefinder unit can be lifted out of the camera, you take out the semi-transparent mirror in the middle (Which is also the reason it's so blue-ish, afterwards it's crystal clear), then saw off the right side of the unit (The frame line / rangefinder parts) so you only have the viewfinder left. It sounds more complicated then it is, and after that you can use whatever material you please to cover it up in a neat little package and add a hotshoe.