Today I brought the KAGRA spare viewport that was sent together with the 2 dirty ETMY viewports.

It seems to be 10 cm diameter and 1 cm thick.

I tried to install it inside the holder for the shinkosha evaluation plate (similar thickness) BUT

- only one stabilizing screw can be used making me afraid that the viewport might move during a measurement

- from the lowest point of the mirror up to ~3.5 cm will be hidden by the holder.

I guess a possible quick fix could be to buy ~6cm stabilizing screw and use the TAMA size holder or drill new holes in the SHINKOSHA holder to move the screws and metallic rode to a good position.