Greetings!
Well... the question is, what do you want to do later with your Aspis?!
If you want to cary your Aspis in your living room around, you can work with wood. For my sake, paint the wooden porpax with bronzemetall color and youre done.
I would never do this, i preffer historicaly correctness
But:
The thing is, you have to attach your porpax very tight to the Aspis. Expecialy when your aspis is heavy (8-9 Kilogram).
Its easy to fasten your (bronze) porpax with nails, on the upper side of your aspis with bended naills. Heres a nice site:
http://www.larp.com/hoplite/hoplon.html
A wooden porpax has to be relative thick (uncorrect) in comparisson to a bronze/Brass to be able to work under pressure. And a shield among other things, in a classical hoplite xiphos/kopis-fight, has to be very resilient.
I saw some beautyfull preserved porpaxes in the museum of delphi (greece). But theyre all heavy and made of thick branze.
If the resilient point is not relevant for you, i suggest you try other metal stripes like a steel plate/copper and bend them to a greek Omega (Ω)-shape.
Heres another one nice manual. (Sadly) in german but very illustrated:
http://hetairoi.de/pages/Porpax/porpax.html
Cheers!
PS: I forgot to say: The porpax made on the second link(german site), is a panel sheet.