Here it is, the 2U rack mountable for 21 Raspberry Pi 3B+. This the rack that held the 1024 Raspberry Pi at CodeOne San Francisco 2019 in the Raspberry Pi Super Computer.

You will need the following:

1. (21) Tapered Heat-Set Inserts for Plastic, Brass, M2.5 x 0.45 mm Thread Size, 3.4 mm Installed Length, Packs of 100. I ordered 94180A321 from McMaster-Carr
2. (21) Button Head Hex Drive Screw, Passivated 18-8 Stainless Steel, M2.5 x 0.45 mm Thread, 5mm Long, Packs of 25. I ordered 92095A457 from McMaster-Carr
3. (4) 10-32 3/8in long screws with flat head. I ordered 91253A001 from McMaster-Carr
4. (1) 18" 80/20 Seris 15 .5in x 1in https://www.amazon.com/gp/product/B00IWZFPCS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
5. (21) Raspberry Pi
6. (21) 90 degree micro USB cables
7. (1) 24 port or larger switch
8. (1) 24 port or larger 5v USB power supply, for example: https://www.amazon.com/gp/product/B07MB12XZ4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
9. And various tools

NOTE: Before ordering the items above, please double check to verify everything looks good, it was a while ago when I ordered these and while I went back through my receipts and I'm confident these are the items, there could be some mistakes. If there are, or there are questions, please let me know.

Steps:

1. Print (21) of the Pi2UHolder.stl files
2. Print (1) of Pi2ULeftBracket.stl
3. Print (1) of Pi2URigthBracket.stl
4. Insert a threaded insert into each of the holes in the Pi2UHolder.stl
5. Put the Raspberry Pi on top of the Pi2UHolder.stl, push on the 2 bottom holes and insert the bolt through the top hole and tighten into the threaded insert.
6. The 18in 80/20 is a little too long. You need to cut off 16.35mm. The most accurate way of doing this is with a mill.
7. Tap the 2 holes on either side of the 80/20 to 10-32.
8. Push the 80/20 into Pi2ULeftBracket.stl, and bolt in the two 10-32 bolts
9. Slid on (21) Pi2UHolder.stl with Raspberry Pi attached from step 5 above
10. Push the Pi2URigthBracket.stl onto the remaining bit of 80/20
11. Viola!