At Reddit, mtietje posted this remarkable photo of their lockdown project: an iPad stand made using an old Macintosh SE/30. The display size is perfect for the 9.7" iPad, and now that iPadOS can use mice they work much better as "normal" computers.
Se/30 shells turn up on eBay now and again. One's there now with bidding at $15. Shipping tends to really bloat the price of even a broken model with the bits still inside. I doubt you could reasonably shave down the SE/30 display housing to fit an iPad Pro, unfortunately: you'd lose all the beveling.
Now, the iMac G4 (below) came with a 15" screen that a 12.7" iPad Pro might hack into without looking too small. Maybe you could even just remove the display entirely and magnet the iPad on the arm?