I just went down this path a couple of weeks ago. Unfortunately, I couldn't find the type of aluminum (t-slot) I wanted at my local home depot so I resorted to Amazon.

I wanted t-slot aluminum because it makes it easier to line things up and correct mistakes vs Brazing/riveting/taping the aluminum together. I got my inspiration by looking at the Veddah frames. The added benefit with t-slots is you can stack them pretty easily

The reason I didn't just buy a Veddah, is because they were sold out everywhere, and I wanted to make mine a little wider so I could get more spacing between my 8 GPUs.

I've posted my parts list below if ordering from Amazon is an option. Hope this helps!

Frame Image: https://i.imgur.com/20h9A1p.jpg

With cards Image: https://i.imgur.com/oCqUVOq.jpg

Tap Magic: https://www.amazon.com/gp/product/B00065VEP4/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

M3 Taps for motherboard standoffs: https://www.amazon.com/gp/product/B07WN8BGWJ/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

(you will also probably want 6-32 taps for screwing in your gpu's, I already had these so i don't have an amazon link)

Framing (I went 1000mm wide, 400mm height + depth: https://www.amazon.com/gp/product/B084C217FD/ref=ppx_yo_dt_b_asin_title_o06_s01?ie=UTF8&psc=1

Used these to link everything together: https://www.amazon.com/gp/product/B07JBX2NDL/ref=ppx_yo_dt_b_asin_title_o06_s07?ie=UTF8&psc=1

I tapped the GPUs into this (Cut it to match 1000mm length): https://www.onlinemetals.com/en/buy/anodized-aiuminum/0-75-x-0-75-x-0-0625-anodized-aluminum-angle-6063-t5-black/pid/24936

I used these to connect the piece above to the t-slot rails + hold plexiglass: https://www.amazon.com/gp/product/B08CB2P2GJ/ref=ppx_yo_dt_b_asin_title_o06_s02?ie=UTF8&th=1

Cut plexiglass to hold psu's + ssd in place: https://www.amazon.com/gp/product/B089P3FMSV/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Just too pretty up the ends: https://www.amazon.com/gp/product/B07CTMKCPZ/ref=ppx_yo_dt_b_asin_title_o06_s05?ie=UTF8&psc=1