Perhaps John will jump into this discussion. He has been doing a lot of work with these cameras as have several other students; myself included.
We are finding very few if any leaks; feeling perhaps it is flare. At least so far. But this is a small sample
I have modified mine to use with 35 mm film; so the image spills over into the rebate. Because of this use I do have the frame indicator taped over, but that is all. No leak issues.
With the holga in 120 mode there is a issue with the hot shoe placement which will need taping. Some type of hole that can be a light problem. With using the mask; i.e. 645 frame, that does not come into play.,
There is a special look to this images and they are great fun as it really frees your mind from everything except the image . Interesting dynamtic.
Check Leon's work with Holga. Wonderful stuff.
Diane's do have some serious leak problems as they were the fore runner for toy camera work.
Check Jonathan Bailey's website for beautiful diane work. He also uses split toning which really places the images on a different level. Lovely, ethereal work.