I would think that 1/4" plexi would work just fine if properly suported. You would have to build a frame for it with a cap rail. (have you ever tried to set a cup down on a 1/4")
Now for the down side;
Plexi will get hazey over time as it becomes scratched up.
Also, plexi does not let air to through it and as the sun hits it, it will accually heat up!
No privacy. Everybody on the ground will be looking up your bloomers.
I do not know what Code your community has adopted, if any? Most Codes require a guard rail system with no opening greater than (insert your code here).
I have never accepted lattice because, in my opinion, it is an "attractive nuisance" and a climbing hazard. (kids love to try and climb it.)
I would try to match the the house/garage as close as I could.
How are the post installed to your roof?
Is it a flat roof?
Could you build a wall (leaving it up about 4" for drainage), to match the siding? (Kind of castle style) this is my favorite and is very popular these days. Also, you could install GFI'd outlets in the wall.
I would look through some books to get some ideas then ask my inspector what he/she will accept.