I'm putting a 2.3m Sirius dome on a 2.5m raised timber platform.
I've never built anything in my life so I've kept it as simple as possible (but VERY sturdy)!

The drawing of the TOA-150 on the EM-400 is approximately to scale.

The raised platform & timber deck will maximize temperature equilibration and air circulation - a critical consideration for observatory building. It certainly won't be a heat sink like concrete would be!
With some silicone sealant around the bottom to be sure, there'll be no water leaks under the walls either (that could lead to internal mould).