So it looks like the Atto Thunderlink units are still going strong on ebay, and the prices are dropping. I bought a second unit to play with (the first one is used for a SSD) for 35$ including shipping. It was missing the power supply and Thunderbolt cable, but it works with any 12v power supply that has a 2.5mm by 5.5mm barrel plug (same size as used by the AKiTiO Thunder units) and I have (way too many) Thunderbolt cables anyway.

I received the unit yesterday and today I got some spare time to try and make a cleaner eGPU with it than my previous attempt​. From ​my test of the CY Thunderbolt3 to PCIe board​ I have a PCIe x4 to m.2 adapter coupled with a m.2 to PCIe x4 adapter. The latter also has a power connector, so the combination of the two together creates a 90 degree powered riser that supports 4 lanes at Gen2 speeds. Since the Atto Thunderlink is a Thunderbolt1 device, this is sufficient to extract every bit of bandwidth it has, while allowing me to provide extra power to the PCIe slot.

For some high-res images, see here:

Front

Back

Closeup (Riser) - You can see the 4-pin connector on the riser allowing me to feed power directly to the slot, bypassing the Thunderlink PCB. The cost of both the PCIe to m.2 and m.2 to PCIe adapters is about 5$ for both if you're willing to wait for them to be sent from China.

I used a 6-pin to barrel plug adapter to power the Thunderlink PCB, and a fed power to the extra power plug on my riser with another cable. The PSU was a regular ATX unit. I installed the Quadro K2200 into the riser, hooked up a 2m Thunderbolt cable to the Apple TB3 to TB adapter and connected this contraption to my ZBook.

Everything worked flawlessly: The card was detected, the drivers picked it up and the bandwidth to the eGPU is the same as other TB1 setups:

At a cost of about 50$ (including a Thunderbolt cable) this compares favorably to the GDC Beast units for cost, while offering a potential eGPU solution for those of us with TB1 laptops or SFF PCs. This is especially attractive for older Macs, the Lenovo W540 and the Lenovo T430s as a low-cost eGPU solution. Just like the Beast, a power supply is a necessary additional purchase, while a case is optional (With some Dremel work, the Thunderlink itself can be slightly modified to serve as a pretty good stand for the setup). A Dell DA-2 power brick can be used for sub-200W video cards if you can put together a basic cabling job to split its 8-pin output into a barrel plug and a 6/8-pin PCIe power plug. This is similar to the mods that we used to power AKiTiO Thunder2 units back in the day.

If you are interested in procuring one of these for yourself, find recycler type sellers that offer them with a Best Offer option and send an offer in the 30-50$ range.