From another angle.

With the blanking piece removed I could mark out the centre line position of the mast well. From here it was a case of building up semi-circular shapes to represent the mast well formers. For this I started by cutting out two 22mm diameter discs from 0.5mm plastic sheet. I used a compass cutter for this job. When they were both complete I marked out the centre line of each then marked out a cut line 0.5mm on each side of the centre line to allow for the thickness of the blanking piece. By this process I produced 4 partially semi-circular discs which are to be used as formers for the mast well.

The attached photo shows the first two formers glued in place along the pre-marked mast well centre line.

The photo shows some detail added to the lower former. This detail would in fact be a mast well drain where the water ingress from the periscope being lowered would collect and then pumped out later.

Another photo showing all four formers fitted.

I then cut out a piece of 0.25mm thick, plastic sheet to wrap around the formers to make the mast well housing. The attached photo shows the 0.25mm thick sheet, cut and fixed in place.

Another photo showing the sheet wrapped around the formers.

To represent the seams on the housing, I cut and glued four equally spaced 1.5mm half round strips around it's diameter. This photo shows the first seam in place.

From another angle. At this point it's really starting to take shape.

Looking up from underneath.

All four panels in place. These were made from 0.5mm thick plastic sheet, cut to size and then bent to match the curve of the mast well before fixing.