My solution has worked well for me, I used a Larsen "Secure Elegance" storm door on mine, it contains the five items that were important for me...
1. Full view window top to bottom...
I wanted a big window with a clear view so when driving I did not have hampered visibility.
2. Tempered Safety Glass (as required by DOT)...
It also had to be made with the right materials so that if I ever got caught up in a DOT inspection, this thing would pass. The glass is double tempered safety glass with a plastic membrane in the middle... same thing as an automotive windshield. You can break it, and the glass will shatter in a gazillion pieces, but that membrane holds it all together, and you can't actually get through it. You can beat on it all day long with a baseball bat, you'll smash the glass but you are not getting in.
3. Dual secure locking mechanism (as required by DOT)...
This was part of the security feature of this door, it had not only handle like all storm doors do, but if you lift up on this handle two 'claw like' latches swing out & down and work like a pair of top & bottom dead bolts with hooks on them.
4. Very difficult to break in, this is a security door...
You can't open this door with those security latches, especially when you consider the bus door jam is made of steel and not wood.
5. A perfect fit...
I had a 32" opening, so I had to find something that would fit... I did have to fill in above the door, I used some 3/4" plywood and then mounted the outside light on that. The door comes in 36" & 32" sizes and can be purchased with left or right hinges...
I bought this door at Lowes, cost me a little over $300. My alternative was to use an RV style door, but I wanted more glass than those have.
http://www.larsondoors.com/storm_doors/products/349_17