Those aren't my tyranids but I have indeed based on ovals.
I got some 40 by 12mm oval bases laser cut in 1.5mm high acrylic (to match the old Epic bases height) as a custom order. There's still enough space I find - I've managed to squeeze 20 guants onto one 40 by 12mm base. I also have a dozen or more other shapes and sizes made. I've also made a few other oval shapes - I buy Titanicus titans to use for Epic but base them on 1.5mm high laser cut bases of their usual sizes.
My laser cutter seems to have gone out of business sadly but I expect it should be feasible to find a different one willing to do custom bases.
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Excuse blurry photo, my current's cheap mobile's camera is utter garbage:
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