Some people take the old cases from defunct macs and modify them for modern hardware. Here's a shop [0] that sells conversion kits for them.

Here are some "sleepers" that people have built over the years [1,2,3]

[0] https://thelaserhive.com/product-category/powermac-g4-conver...

[1] https://old.reddit.com/r/sleeperbattlestations/comments/1gva...

[2] https://old.reddit.com/r/sleeperbattlestations/comments/1871...

[3] https://old.reddit.com/r/sleeperbattlestations/comments/t7tz...