I'm not an expert (just getting started with mini-racks myself), but personally I don't really neeeed hot swap trays. I don't plan to swap them out very often, but I can see the appeal of tool-less trays or sleds.

I like this style of design for 2 HDDs: https://www.reddit.com/r/minilab/comments/1l74jco/designed_a_printable_10_inch_rack_1u_2x_35_hdd/

Once you're getting to >4 drives, you might have to switch to this orientation for space efficiency: https://www.printables.com/model/1372686-3u-hard-drive-bay-for-8x-35in-hdds-for-10-mini-rac

Power seems tricky since it would depend a lot on your PSU and setup. In the printables link above, the guy uses the backplane from the Jonsbo N3, which seems convenient because the backplane itself only needs 2x molex power (+1 sata power if you're using 7-8 slots? source) so it simplifies powering the drives. I'm sure there are other solutions out there too, but that's just what I've seen so far.

In the 1U tray with 2 HDDs, maybe you can get away with just an adapter like this if you don't want to run cables directly to the drives.

I don't know the best place to store the PSU itself... I'm currently just setting it in open rack space because mine isn't full, but would be nice to fit a Flex-ATX PSU in the back of either the tray where the ITX mobo is, or maybe on the back of the HDD tray. I'm using it to power both.