I have managed to mount a Seiko copal 1 shutter on to the body of a Holga.
It's indeed possible but it required a fair amount experimenting and of grinding with a dremel tool.

As it turns out, the section between the lens and the body is approximately the same width as a copal 1 lens (and also a copal 0)
I removed the lens and then that middle section. I used a hole saw to cut a 44 mm hole in the Holga body (which required further grinding with the dremel), so the copal 1 shutter would fit. The inside of the body needs much grinding- removing the rest of the raised square wall around the original square opening.
The inner section at the top also had to be removed as it was in the way of the shutter. Half of that section was cut out with an exacto knive. The whole which was left need to be made light tight with some black plastic.

A 42mm-52 mm step up ring was used to attach the holga lens to the shutter. The thread of the Sieko shutter front is 42mm. The Holga lens barrel needed to be ground down a little (it has a notched pattern- so just grind that down) to the point in which the barred could be force-fit into the 52mm side of the step up ring. The step up ring can be simply screwed into the front of the shutter.

The result is pretty good. The lens is slightly too far from the film plane though. It doesnít quite focus at infinity. I did not even attempt to create a lens which can change focus. Iím happy if I can get from 12ft Ė infinity in focus!
As you can see from the attached picture, Iím not getting that yet. So I think shaving 1mm or so off the end of the lens barrel would help to bring the lens closer in.

I did try using a copal 0 shutter originally but the maximum aperture was not quite wide enough and there was fairly serious vignetting. Iíve attached a picture which was taken with the copal 0 shutter.

The images donít have the Holga vignetting. I think the vignetting is a result of that inner square box around the original square opening? I do miss the vignetting and would like to recreate that look some how? However, I could just burn it in in the darkroom (as I shoot primarily B&W)

The other issue is that now that the middle section is gone, so is the tripod mount! The reason of this project was to shoot with a tripod with lower shutter speeds (without just using B mode).
So, I'll have to figure out how to install a new tripod mount on the bottom of the camera body.

The picture of the deck in snow is taken with the copal 1 shutter and the other with the vignetting is taken with the copal 0.