The way the inside frame will be set up on the inside of the cabin side and how in the inner ring end up protruding though the cabin side .The mastic would be used be on for the inner stainless frame and seal against the inner cabin side.
Where it comes through to the out side of the cabin I will be using a different sealant sika flex
I placed the new port light into the old port light cut out just to work out what need to be done. So Donks mastic will be on the inside of the cabin once that is fixed into position the teak frame and out port light ring will be bought up to fit the inside and I will use a sika flex product to seal the out ring and teak frame to the cabin side and teak frame will be fixed from the inside of the cabin as well as the sika flex sealant on the out side.
I dont think mIll have any problems.
My main concern is the angle of the outer ring will it comect water either rain or sea water and when the port is opened if it had this water will drain inside . So I will try as much as possible to rectify this by tapering the teak frame so its more up right and reduces the risk of water drain . That will be the hardest thing to do as the cabin side is not at 90 degress. It will be a trade off with ascetics and bloody water draining inside when the port light is opened
Note the outer ring slides over the inner ring when the two part are joined so there is room for adjustment to assist drainage
If you can visualize a teak frame around the stainless steel frame in the above photo that is 70 mm wide this is what I am doing . I think in the end Ill make four separate teak frames Identical. These teak frames will be 30 mm thick with the outer ring inserted
into them
outer ring with out a teak frame. These are 50 mm wide and will be reduced to fit my boat . Once Ive made the teak frames
I hope some of this makes sense

Initially I was going to use one piece of teak with two port light cut outs but feel this might be a problem as the cabin side are not a flat straight surface so four teak cut out make sense.
If I end up with color matching problems after Ive filled in the old port light holes I might paint a panel surround the covers the port light area . I will be painting the non skid areas this spring with a light Grey, so I could match this on the port light panel save me trying to match up the original color on the cabin top. or paint the complete cabin. Which I really don't want to do as it in reasonable condition.