"cheapest" rigid vertical wall?

Hello,

I am making a test model to measure and correct printer skew.
This one works for me fine, but I wonder if there is a good way to improve it making faster to print.

These are two 4mm thick walls; but even with min infill and single-layered it takes 2+ hr to print.

The purpose is to make a rigid vertical square for further measurements and its only requirement is to be rigid enough to be printed straight with no degradation during print.

Any ideas welcome.

TIA

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