To update on the PME90 with Arax cup option: it actually does NOT fit. I took a chance on it as to be clear, they state just about all Hasselblad finders except PME90. I don't have the non-metered PM90, so was hoping it would be same, but no (and am not so sure this will work on the late PM90 either). Alll earlier prism finders look different in the area, so worth a try if one needs a new one.
So Arax cup will kind of stay in place over eye piece on PME90, but not fitting in as it should. It's usable, but clearly not the right thing for it. Eye piece ring has a rather thin groove and original cup is not only fitting right inside that groove, but also has a soft skirt all around, which appears to be keeping dust out of the area and keeps eye piece steady (without rubber cup, eye piece ring has a little side play).
So for now I'm left with basically rubber cover without flip out option. Will try rubber cement when I get to it, but not holding high hopes for it as rubber itself is very thin and glueing it back together will be challenging. It may help holding the piece as one, but I'm not sure flipping it will work. I'll get back on this.