The pan: build a form with the drain pipe sticking up the middle of the frame. pour in pure mortar or concrete into this form and create a bowl shape for the water to run from all sides to the drain. I would use 2 x 6's for my form and have the concrete all the way to the top of the forms on the sides and taper to the drain height in the center.
The framing of the enclosure would be like building a free standing square closet. I would use 2x4 studs, insulation could be a factor depending on where you live and how you plan to protect the water in the faucet pipes during winter.
Completely line the inside with cementious wall board. Then line with tile.
If you want to spend more money but have less of a job, purchase a seamless fiberglass enclosure and just slide that baby into the frame and walla. Here you even have your own pan.
I would also have what I call soffett vents up in the wall area by the roof line for your condensation to move out. Seal over during winter.
Have a "hip roof" over this baby, have a good 1 foot overhang to protect the vents and allow water to drip down and over the establishment.
Side the outside with a good masonite siding, paint and caulk. use 1x4's for the sides and corners to frame it out. But a cheap metal skinned door for the thing. Install a lock for it when on vacation to keep people out.
easy as pie.
Ps, install a ceiling can light with protection in the baby for night use.