I think it's a good start. Nice solid floor, but you will NEED to use some clamps to hold it on before you start putting things up there. Any plans for sides, or is it going to be kept as a platform?

i agree with DJKnightmare, it is a good start. if its just up there to see how it fits and how it looks, then zip ties are fine, or even to use them to hold it while you clamp or bolt it down that works to, but if its there to stay zip ties are not a safe way to do it. zip ties will dry rot in no time at all in the sun, become brittle and break as soon as any pressure is put on them.
a lot of people here posting how stupid it is or how dumb it is, you know dont you think a little constructive help would be better then just bashing the op for his idea, just my two cents.

You could've just cut a ladder in 1/2 and done the same thing. For convenience at the car wash, you could use velcro instead of the zip ties so it's easier to remove and reinstall. NOT!

It might be inexpesive right now, but wait how costly it get when you put stuff up there, flies off, causes an accident and you're getting sued. It won't be so inexpensive then. Cool that you thought of doing your own thing, but make it safe!