The new S-Wind sign is everything you've been asking for.
A portable sign with one or two clear wind covers, heavy duty track and 0.060" letters. 
The sign is on stands for easy placement and portability. FOB KY. Due to a shortage of clear poly and acrylic products there may be some delay to make any clear shield doors for signs. 
Letter storage case option FOB IL. Color letters option ship separately FOB AL. Letter Storage cabinet offer is $119.90 (reg. $149.00 add for wheel option if desired, plus separate shipping added after order is charged). GET FREIGHT QUOTE HERE

Additional information from customer questions:
The frame contains lighting inside the cabinet which is visible from both sides. 
Both sides have white "faces" with track so a message may be easily arranged and readily changed.
The overall size of the sign cabinet is 48" tall, 96" wide, plus the height added by the adjustable height steel stands as shown.
The doors lock closed and are designed to keep the wind and busy hands from changing the messages. 
The door(s) do not create a water-tight closure, as the entire sign, cabinet, faces and letters are not sensitive to moisture and do not require such expensive upgrades.
Letters are 8" tall, printed onto 9" tall clear plastic. They are pushed up into the top track, then rest in the bottom track, secured by the top track.
The entire sign face (on each side) accommodates 4 lines of text or custom-made optional word/business name panels.

The sign takes an average of 12-15 work days to manufacture once the order is received. The cost of the sign is for pickup at our Kentucky plant dock. Once the sign is ready, we ask you to call to arrange for pickup. You will need a full-size pickup or flatbed truck or trailer. Custom sign orders are not cancelable.