High performance, small form factor, no graphics

I was pretty big into SFF systems myself a few years back.

I am not aware of any production chassis on the market that will be as small as a NUC so you'll likely need to consider 3D printing if you plan to truly go that small, especially if you plan to use an AC power adapter to save internal space. Luckily, Thin ITX boards exist to accommodate this quite well, though not with your processor of choice. I cannot think of a single Thin ITX motherboard available that will support an 11900K, most of them top out at 65W. If you want to go that route, here are a couple of boards to consider: https://www.gigabyte.com/us/Motherboard/GA-IMB410N-rev-10/support#support-cpu, https://www.asrock.com/mb/Intel/B460TM-ITX/index.asp#CPU. Keep in mind they do not support 11th generation and likely never will with the limited H410 chipset and low adoption rate of Thin ITX boards in general, but the Core i9 10900 and 10900T both have more cores than the 11900 at a slightly lower clock speed, so if your work can be parallelized by additional threads, going with the 10th gen CPU would be a better bet anyways. For cooling, a stock Intel cooler will suffice for a 65W or less CPU, but if you want to get fancy, grab the Cryorig C7. Arguably one of the best ITX air coolers on the market. Sadly we don't carry it, but it's around online and will definitely get the job done.

If you want to avoid the limited board support and go with a larger standard ITX board and traditional memory, you can pick pretty much any ITX motherboard on the market and be set. If you still wish to keep internal volume down, you can use a DC converter like the HDPlex 400W and an external power adapter from a gaming notebook: https://hdplex.com/hdplex-400w-hi-fi-dc-atx-power-supply-16v-24v-wide-range-voltage-input.html. While this still won't be NUC size, it will definitely allow you to go much smaller than traditional ITX builds with internal PSU's.

If you need some inspiration as to what some good 3D printed ITX cases look like, this one is one of my favorites: https://www.thingiverse.com/thing:1849483. Good luck with this project, sounds like a fun one.

I don't believe we carry any 1U chassis in our stores that would work for this build idea, you may need to source that elsewhere, but we should carry most of the other components you'd need. If you are going with a 1U chassis, you should be able to get away with running a Flex ATX PSU: https://www.microcenter.com/product/457426/solid-gear-flex-320-watt-mini-itx-non-modular-power-supply. Do keep in mind that our selection of Flex ATX PSU's are quite limited and this 320W unit isn't exactly ideal for pushing high-current. With a single 4-pin ATX connector, you're limited to 150W for the CPU which is super easy to exceed on a 10900K/11900K, especially if you partake in AVX workloads.

Silverstone makes a 350W Flex ATX PSU with an ATX 80+ Gold rating and includes the 8-pin EPS connector allowing up to 300W of power to be pulled through the CPU: https://www.silverstonetek.com/product.php?pid=784&area=en. If you can find this for sale anywhere, it would be my recommendation. If not, try to find FSP's FSP400-60FGGBA. It's a 400W gold-rated Flex ATX PSU with an 8-pin EPS as well. Lacks the cable sleeving and warranty support Silverstone has going for it, but it's still a fine unit.

Aside from that, you should be able to get the motherboard, CPU, RAM & cooler at our store without an issue. If you are looking for recommendations on board/memory selection, I can help with that, but I'll need to know more about your overall budget and specific features you are interested in. For the 1U chassis height, I'd recommend picking up the Noctua L9i: https://www.microcenter.com/product/617825/noctua-nh-l9i-low-profile-cpu-cooler or Cryorig C7 I mentioned before if you can find it anywhere. Both will out-perform a stock Intel cooler and you'll likely need the better cooling performance in the tighter 1U configuration.

I did have a question about all of this. Are you going to be mounting the 1U rack in a cabinet? If so, are you going to have it tightly condensed between other racks? Or will there be some space in between? It might be worth cutting a small hole in the rack panel and cover it with some fine mesh to allow for better intake for your CPU cooler. Might go a long way with maintaining your boost clocks under longer load periods. Definitely looking forward to seeing this project take shape!