What I have
I have an old Acer Aspire A515-51G and I also have a desire to have a home server. In combination this creates a need for the laptop to carry more HDDs than it can from factory. Usually it can take one M.2 SSD and one 2.5" SATA drive.
What I want
I want to make the laptop able to run at least 4 HDDs in RAID 5 for storage and have one SSD as boot drive.
What I did
The buying phase
- 4 HDDs
- M.2 to SATA adapter
- 12V/5V SATA power converter
- 12V 10A power supply

The designing phase
I needed space to hold the HDDs, so I designed a 3D printed a cage to attach to the bottom of my laptop. Two 80mm fans can be attach to it. You can see it on printables. Also I remixed a tray (here is the original) so the HDDs can slide in and out of the cage.
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The assembly phase
A cutout at the bottom of the laptop was needed to give access to the SATA ports of the adapter.

I put a 2.5" SSD into the already present place for it in the laptop.
Then the cage was attached to the laptop, the HDDs were put in their trays, the trays inside the cage and the cables attached to the adapter and HDDs.

I actually have a similar setup that started out just as janky!
This is an HP mini PC I found on Ebay, with an m.2 to 6x sata adapter, hole dremmeled through the top of the case with a custom printed I/o shield to stop the cables or my fingers getting sliced on the metal.
After about 2 years of it sitting in my cupboard (like the first pic), I decided to print myself a LabRax case for everything and make it cleaner, still the same setup inside though!
Before:
After:
That looks very clean. Though I wouls describe my setup as middle ground between your first and second pricture. I have a defines place for everything, just a few open cables.
What do you use to provide power to your drives?