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$150
This converter mounts on top of your desk, and you push it (it's not electronic) up and down to adjust from seated to standing position. It's helped my posture and my back.
ErgoTron WorkFit-S, Single HD with Worksurface+ standing desk attachment, barely used. Easily mounts on any desk. Black. $200 OBO. New one sells for over $400.
https://www.ergotron.com/en-us/products/product-details/33-344#/
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