Depending on the amount of money you'll want to spend, here are a few ideas for a 'cheap' folding rig:
1- Get a cheap $20 shoe rack that will host your rig:
https://www.amazon.com/gp/product/B00KNBRV6C/
I got the expresso, because it's already painted. Keeps the buggers away, and the $1 extra for the painted version, saves you about $8 on spray paint, or Wood Waterproof/Sealer.
Each stage can hold an ATX motherboard and power supply, along with 3 GPUs plugged in the motherboard, as well as an additional one on a riser.
However, without fortifying the rack, I would only recommend up to 3 systems (one per floor). That is 1x Mobo + RAM/CPU/SSD + 3GPUs mounted on the mobo + 1GPU on risers + 2 PSUs if necessary.
You can mount the GPU's on risers with Zip-ties on the spokes, if you like.
The rack is just high enough for mobo + GPUs (about 5.25" between levels).
2- Get a $120 motherboard equipped with 3x full size PCIE slots. Any one that your CPU supports.3- Get matching CPU (6 core for 6 GPUs, quad core for 4 GPUs, dual core running at least 4Ghz with HT for 3 GPUs for linux; or take 1 GPU away for Windows, as Win need an extra core to run), and RAM.4- Get RTX 2060 an RTX 2060 Super card (costs 13% more than a regular RTX 2060 for ~15-20% performance improvements), then a regular 2060, then a 2070 Super, last 2080 Ti for highest score. Don't bother getting a regular 2070 or 2080, unless you can get them for $350-399 for the 2070, or 399-599 for the 2080 (second hand). The Super cards are much faster for the dollar.5- Cooling, get a simple home fan.
For up to 6 GPUs, get a $14 fan (like this honeywell):
https://www.amazon.com/gp/product/B001R1RXUG/
Face it towards the 3 GPUs in the full size slot on the motherboard. Forget about watercooling. It is less efficient, and can leak and destroy your pc.
The remaining GPUs can have sufficient cooling when spaced apart on a next level.
For more than 6 GPUs, get a $18 BOX fan like this:
https://www.amazon.com/gp/product/B00I8Q3082/
The 20" BOX fan is tall enough to provide cooling for 3 of 4 stages in the shoe rack.
This box fan has a power lead on the back. It's pretty easy to cut a small hole in the center of a 20 by 20" paper AC filter, and mount the AC air filter to the back of this fan with duct tape, or some other mounting hardware.
Use as many full size slots as possible for your fast GPUs.
If you have any remaining GPUs, use them with risers in the PCIE 1x slots.
You can mount them on the second stage, if they no longer fit on the first stage..
A single shoe rack like that can host up to 3 motherboard systems with up to 5-6 GPUs each.
Or, you could mount 2 motherboards (one on the 4th level, one on the 1st level), and feed about 6 GPUs per level, PLUS whatever you decide to mount on each motherboard, plus 1 to 2 extra per level.
That's about 22 GPUs max!
Insane! 
You could physically host more than 6 GPUs per level, but heat becomes an issue when they're mounted too close to one another.
Aside from motherboard/CPU/RAM/GPUs and PSU, this $20 shoe rack, and sub $20 FAN has been the best thing for folding since sliced bread!
Especially if you have a balcony, somewhat protected from rain.
If it's not protected from light rain, you can possibly paint and waterproof a cardboard box, and use it as a roof. While the fan is blowing air from the back forward to the edge of the balcony, basically blowing any light rain away from the mobo.
It's good to blow the air away from the windows/doors , towards outside (away from the building) because in summer it'll have the tendency to heat up any room that's facing the balcony, if the air blows towards the door/glass.
You can close off the sides with the cardboard box the shoe rack came with, so the BOX fan focuses most of it's air where it's needed.
However, most of you will run less than 3 GPUs, in which case the small 25Watt honeywell fan is more than enough to supply the cooling, and will be more efficient.
The box fan uses 50 watts of power; but it also cools about 3 times as many GPUs!
post edited by ProDigit - 2019/07/17 14:43:26