I'm planning hanging plants and a Hammock off my observatory... Use the area nicely.
I was going to originally suggest an outside pad for the Dob, but it seemed like you really wanted the Dob in the observatory... I do like the idea of a 2 foot square monolith in the center of the observatory.. Use that for the Dob. You can have it about 2 feet taller than the floor, to give your dob height... Build a Pad for it when you get your other scope and mount...