Hi Matthew,
My old observatory was a 2.7m diameter dome on round ring (timber with steel support) mounted on top of a 3.0m x 2.4m section supported by steel box section and clad in Colorbond flat sheeting. I initially had enormous trouble with sealing the two sections together. It leaked in the same spot as yours - over the keyboard & electronics. I tried Silastic which would work for a few weeks, however it would always harden and shrink - even the outdoor compatible varieties. What eventually worked and stayed weather proof for nearly 17 years (until I sold it and upgraded to a Sirius 3.5m) was "Concrete Foam Expansion Joint" material used as a gigantic gasket between the mating surfaces. To add some protection I sealed the outside exposed compressed gap with Sikaflex. This is more expensive than Silastic however it lasts 10 times longer and remains significantly more flexible
. I painted over the Sikaflex with Wattyl Solaguard to provide final sun protection (same colour as the Colorbond).
Hope this helps.
Cheers,
Chris