Been a while again. Not much has happened though. I've finally started tackling my other obstacle, the deck tank. Considering my mini-lathe is a metal lathe and not a wood lathe, I had to figure a different way than my initial dummy tank from wood.
Current effort was the very un-original build-up of styrene and milliput. Practically I cut a circle of a slightly oversized diameter (28mm). THen I put a cross of two frames with the correct curve on top. And then just fill up the remaining space with milliput. During that process I used an external shaper to push the milliput a bit to get a correct shape and to not end up with too much material on one side and too little on the other.
I also keyed in a 6mm styrene rod in the centre to centre it in the chuck of the lathe.
After this process I machined the milliput to shape and ended with some sanding. The result is acceptable, better than anything I'd achieve by hand. It was also a good exercise with the lathe for me, although I'm aware this is not a conventional way of doing this.
Next challenge is finding a tube of the correct diameter (27mm) to fit between the two caps. I was thinking of the wood one, but that would be difficult to seal and get a smooth surface.
Here you see the two mushrooms with the external shaper.

Here you see the places on one of them where I pushed the external shaper on top to even out the material and get an idea of the real volume of the end-result.

And the first "cap" checked against the external frame (in this case the frame is less perfect than the cap since it was cut by hand):

I've also continued a bit more on the forward part with supports and piping, a preparation to instal the grating over the piping.