It seems it is by no means impossible to form a testudo with oval shields. Perhaps, you just need to huddle closer. You might want to check this pic (and a few ohers) in Fectio's re-enactment site.
And here is a pdf of the scholarly paper on the formation that the re-enactors were replicating. The Internet is indeed, as Ranika says, full of bs., but now and then you do find useful stuff.
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