Your aspis and you, my comfort issue.

[Giannis K. Hoplite]

Janas, facing a shield in bronze is one of the most difficult tasks for an experienced armourer, and whilst there are at least a couple armourers that can do it very well (manning imperial is one of them), the ancient method of doing it is still somewhat unknown. The ancients used very thin sheet that added very little to the weight and its purposes was mainly structural than protective. In fact it is even possible that a bronze cover weighed less than the two layers of fabric and stucco that the other shields had.

The way to secure a wooden core would be to drill holes in the perimeter of the porpax. right on the edge that is flared out, and nail the wood underneath.
The way to shape the wood would be the make the inside fit your arm as best as possible, and the outside to fit the bronze as best as possible. Leave some room for a leather lining on the inside, and you have a shield that sticks to your arm and moves with your arm without effort.

An alternative would be to add layers of leather until it fits your arm

The shield from manning imperial that you posted was created after discussions i had with Craig about this type of porpax and how they probably had a leather body fixed to them. I admit that this is what i believed at the time, but later we found out that many porpakes have big nails attached to them that show the actual thickness of the wood, and a number of them even had pieces of wood still attached around these nails.
Khaire
Giannis