This place has good deals on door tops, as low as 48 pounds($68 US). I think they are Galvanized with aluminum skin. Shipping is getting to be almost the same these days from UK as across The States, except for bulky items. So, it might be worth it.
www.sp-4x4.com
I'm having a love affair with stainless steel hardware these days. I'm on a mission to replace all rusted hardware with stainless on my Series, so stuff like this doesn't happen again. I would suggest you try to replace whatever posts are in any door top with like stainless threaded rod. I just put new nuts on mine, and had to use the thread gauge on it to get the size. Mine are 7/16" UNF (24 pitch), if that helps. Not sure the length, but you could buy it long and cut to fit. Been getting Stainless hardware on Amazon. 1/8th the price of my local hardware store. I like to support local, but come on Ace, $30 for 6 odd bolts?!
Anyway, I imagine aluminum skin, Galvanized steel frame and stainless post rods would make for an indestructible door top. Just paint it all so those different metals don't react to each other.