Auxilia shields what options are there?

[caiusbeerquitius]

Might be this book:
L. Rossi, Trajan's Column and the Dacian Wars, London, 1971.

Folkert, it might be sensible to specify the period a bit more exactly than within a hundred years timespan.
Anyway, you may build a rectangular or an oval shield. Since the only findings of oval or round shield boards we have from Roman times are dished, you might want to consider building a dished shield. These findings derive from Dura Europos (around 260 AD) Thorsb(j)erg ~(3rd century) and the Fayum (Trier collection).
There is a 1st-century shield from Doncaster, which was found in a quite bad state, and may have been (IMO) either oval or rectangular, but the excavation report is a bit unclear in many ways. The shield is usually, following the excavation report´s suggestion, reconstructed like this:


However, I´d be careful with this reconstruction.
You may want to have a look at this site:
www.romanarmy.net/artshields.htm
and at this article from our very own Peroni & Madoc:
www.vicus.org.uk/documents/Auxiliashield.htm

IMO it might be a good idea to actually use a rectangular shield in an Auxiliary display, just to make something else than everybode else does.
(As is, I´m still missing a group of mid 1stc. AD Legionaries with oval shields... ) If your display is during the Dacian Wars, you may very well use a shield design from Trajan´s column.