I'm looking forward to seeing the details of the transfer and if you're able to find an elegant solution to:
Case power switch wiring
Front panel I/O
If you're able to figure out something that looks good/works well, I may very well go out and purchase a bare MT Motherboard and do the same.
I'm no where near done with the case transfer but here are a few pics to give you an idea. The plastic protective cover on the glass is still on so that's why the side doesn't look completely clear. PSU is not in yet. No front fans yet.
The front I/O is at the far right side of the PSU cover. It's connected up and the audio/mic/USB 2.0 works. The motherboard's USB 3.0 header is connected to the top of the case for easier access. I removed the steel cover over the I/O panel's black/white plastic. I've ordered a 3" black Apple logo that will cover up the Darkflash logo on the PSU shroud. Should look great. Was lucky that the Dell I/O panel stays with the black and white theme too. If you buy the black version of the DLM21 or 22 it will blend in even better at this location.
The Dell OEM power button cable hangs down to the right of the mobo. Blends in with all the other black cables. Just open the glass door to have access to it. You can see all the extra inches of space for a longer gfx card that this case offers. Cards up to 12.5 inches in length will fit in this mATX case. The lower front 120 mm fan will blow air directly over the Aorus RX 570 GPU.
There is a small opening beneath the I/O panel you see above. All the cables fit easily and the 19 pin header plugs into the mobo via a cable routing hole in the PSU shroud. The hinged glass door opens quite easily so it's no problem to access the power button and front I/O. If you will be booting from an NVMe drive you could put Clover on a USB 2.0 flash drive and plug it in internally into one of the two (external) USB 2.0 ports shown above. Also, you could plug in LED lights and use a USB 2.0 port for power.
Here's the Dell power button that lights up white when on and orange when you're getting diagnostic info. I've decided to keep it plugged in for now as I can access it easily and didn't want to cut off the power button to connect the cables. If you do there is no way to get that diagnostic info from the orange LED.
View of the top front case I/O panel. I can connect up the power button later if I want that. For now it's not functional.
If you do want working headphone/mic jacks on the case you can purchase these adapters on Ebay and do just that.
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Here's why the non-working case Mic and Headphone jacks are not a deal breaker for me. I never use a Mic via the 3.5mm jack anyway. The few times a year I use a Microphone it's connected via USB 2.0. I have PC speakers on my desk by Creative. Those have headphone and Aux inputs right on the speaker. Those speakers connect to the rear panel line out 3.5mm jack. So I really don't need either the headphone or mic jack working on the front top of the case.
The lack of a working reset button on the case is also a non-issue for me. Holding down the power button for 4-5 seconds does exactly the same thing.
I like this case because it's not a large ATX case with too much empty space. Has just enough room for everything to fit comfortably. The cable routing for a non-modualr PSU is excellent and you can make a really clean looking build with good airflow. It's not giving you great airflow like the Fractal Design Meshify C but I don't plan on gaming with this system so that's why I opted for this high quality lower cost case.
Here's what the case maker suggests. Which is complete and utter overkill for a Dell Optiplex 9020 transferred to this case. They've got a dual 120mm fan/radiator on the top. Dual 120mm fans in front and a 120mm exhaust fan. All with LED bling. Of course they sell all of that under their Aigo brand. You might possibly need all that cooling with an overclocked 8 core 9900K that was used for triple A gaming with an RTX 2080. For my modest needs it would be a complete waste of money.
Here's an example of a combo liquid and air cooled DLM 21. Note how much interior space these have with a mATX motherboard installed along with three case fans.