Realistically, you have two options for flooring: wood or cement. A poured-cement floor is virtually maintenance free. Make certain you use (or demand) a high-quality grade of cement and a proper mix. I wish I had. The only complaint I had about my observatory was that the contractor who poured the cement floor used far too much sand.
If you use wood for your floor, choose pressure-treated lumber. Elevate the floor to allow sufficient air circulation to prevent mold and mildew. Also, having the structure off the ground slightly is beneficial because rodents won't nest in an unconfined, open space. Allow approximately 1/4-inch spacing between floor boards so cold air doesn't pool in the observatory.
Wood flooring is more forgiving on your feet than cement, especially during long sessions. Make a cement floor "softer" by standing on a reasonably thick (temporary) carpet. Alternately, buy interlocking soft flooring squares available from home centers.
The main consideration for your walls is height. If people are going to be using the observatory (not just a scope-and-camera setup), you want the walls high enough to block out any direct light while still allowing access to a large area of the sky. About 7 feet (2 meters) high seems to work in most locations. For large observatories, you might need slightly higher walls to eliminate direct lights.
Observatory walls, especially those for personal use, vary in thickness. I've seen structures with walls made of stucco, brick, concrete block, tile, shingles, siding, and plastic. For my location in the desert, I chose plastic. Thicker walls retain heat. Some paints and protective overcoatings reflect most light and heat, but, in the desert, coatings abrade rather quickly.
For the outside walls of my observatory, I used an ultraviolet-resistant PVC paneling called PalRuf. I recommend this material. The manufacturer guarantees it for 20 years. The panels are corrugated, approximately 1/8" (3mm) thick, and measure 26" by 96" (0.66m by 2.4m). You can cut a panel easily with a pair of utility scissors. I covered the edges of the walls with a galvanized ridge cap.
I connected the panels to a wood understructure (made of 2x4 lumber) with roofing screws, which self-seal via attached neoprene washers. The panels reduced solar heating, a major consideration in the desert. Some PVC protects against ultraviolet light only on one side. Install that side facing outward.
I used white PalRuf for the walls, but I painted the observatory's interior flat black. When I stayed inside the observatory for more than 15 minutes, I boosted my visual limit by 0.5 magnitude. I'm certain the black walls played a major role in this.
The next choice you have is whether or not to apply a facing material to the interior walls. If you don't (my recommendation), you can use the wood studs to support shelving, run wires, attach lights, and more. The only reason to finish the interior walls is for appearance.
Next week, more about your observatory's interior.