After reading about Barry's excellent DIY mini dome, I decided to have a go at building one for my Ikelite setup by combining an old dome port with an old flat port. The rear end of the flat port would be used to mate with the housing, while the central portion of the 6 inch dome port would provide the curved front surface of the mini dome.
After some calculations/measurements I used a fine hacksaw to cut through the barrel of the flat port and smoothed the cut surface using some fine wet and dry paper on a flat surface. I then marked out a circle on centre of the dome, with diameter slightly larger than the external diameter of the barrel of the flat port (which is 4 inches). I cut out this 'circle' using about a dozen hacksaw cuts, and smoothed the cut edges of the dome, again by rubbing it on wet and dry paper on a flat surface. When the external diameter of my new dome was equal to the external diameter of the barrel (ie 4 inches), the two cut surfaces were glued together using acrylic cement.
Once the cement had dried I painted the barrel inside and out with flat black paint to cut down on reflection. I made a port shade out of a short piece of PVC drain pipe, re-sized for a snug fit then painted flat black.
Now, while the outer barrel diameter of this port might be 4 inches, the dome radius is 6 inches and I note Barry's dome has a 4 inch radius. This doesn't seem to have created a significant problem for me apart from the fact that it was designed for a Tokina 10-17 with 1.4 teleconverter, and it won't work with the Tokina on its own (at its widest the lens takes in the edges of the port). But a big plus is that it cost virtually nothing to make, and another advantage I found is that the existing Ikelite zoom gear works fine with the Tokina fisheye plus TC.
Sorry I didn't take a few photos of the construction phase. I've since ordered a 4 inch acrylic dome that I hope to use to make another mini dome port for the Tokina on its own, so if anyone's interested I'll post a few pics of the steps in that.