Here is what I did. I spent a long time thinking about ways to do this and made many mistakes. The pad was my first. I tried to save money by having it poured at the same time as another slab was laid. Next time, I'd make the slab 3 meters square not four, and have it stand proud of the soil. Much less opportunity for water to get under the walls. I would also get the pipe for the pier positioned before the slab was laid, rather than rely on reo and a box of concrete to attach the pier, I'd buy it a meter longer and bury it in the slab footings. I think I probably went overkill on the central concrete column, but there is no way to tell.
I had a couple of data pipes laid underground, and got the sparky to connect the power and internet/phone line.
After seeing my workshop built, I had the idea of building a three meter cube with the same method. The garage people said their engineers weren't happy about making a building and not a roof. But they were persuaded and It was a terrific solution. I put up the framing for three of the walls and then built a scaffold to hold the joists in place. Once the joists were in position I slid the floor up onto the joists attached it securely and cut the floor hatch out. The floor is just above the doorway so the top of the wall is about hip height. I then fixed a drilled steel plate to the pier, aligned it roughly N/S and welded it on. The main plate duplicates the shape of my tripod but made of a substantial slab of steel. I hadn't taken into account a protruding part of my CGEM mount, so I had enormous fun, making sparks and grinding some of the slab away.
I then had to think about the top. I couldn't get anyone to respond when I tried to buy the domes made around here and couldn't afford the one other I looked at. ($11K)
My first turret was made of ply. Heavy exterior ply. I had a building designer friend tell me where I went wrong So I pulled it apart and rebuilt it.
I went through a lot of ideas trying to work out how to do a cut the friction as the turret rotates. Eventually I found these little Chinese rollers that are used in screen door applications and ordered 600, they are rated at 20Kg each and the turret weighed 350kg.
A mate has a truck with a crane so he loaded
the turret onto his truck and put it in place. It rotated, but it was just too hard to push. I had no alternative but to scrap the turret and do something else.
I had built a little 3D model that showed me I had the proportions right. (I got that idea from Medieval cathedral builders!) So I took the turret to a couple of metal fabricators and gave one the job. They did a great job and it is now on the cube and rotating well. In the next few days, I'll get my scope together, carpet both floors, put some furniture in and open the hatch to space.