After about a month of measuring, drafting, measuring, modeling, ordering, measuring, and shipping a new backplate showed up for my machine today. For installing a micro ATX based motherboard (and other gizmos) inside.
I used 20mm motherboard standoffs for power cable management. This set where the micro ATX I/O panel and PCI cutouts needed to go.
Two 92mm fans fit on the back. One of the 92mm sections was made big enough for an Asetek 545 LC liquid cooler. So one's installed. The fan for it isn't on yet -- that's later.
The Altair's DB-25 serial ports are still there (natch!) There *should be* room for the cassette interface under the motherboard. But I don't have one (yet). So the audio jacks and DB-25 for that are already cutout.
I have a 5-bay 3.5" Hard Drive Cage to put in for bulk cold storage. That's also later.
I'm still waiting on an ATX power supply and a 5v-12v boost converter -- I'll be using the 5v standby voltage from the ATX power supply boosted to 12v for the Altair Clone. You can draw something like 3A through the standby and the Altair needs a whopping 40mA, so I think that'll do fine. I didn't want to order the power system until the backplate showed up and... you know... fit.
The motherboard installed right now is fried and not permanent. So once the power and cooling is in place the Altair will go back on its shelf and do its thing until the computer currently acting as a terminal needs replaced. Then its time to buy motherboard, processor, RAM, and video card.
Once everything is all installed and happy, I plan to share my dimension drawings. It's not perfect (thank God for ATX wiggle room), but if anyone else is interested I'd like to share the fruits of my labors -- assuming the power supply fits. This wasn't the cheapest thing in the world to get cut and bent, but the shop I went through offers discounts on volume orders. So if there's interest I may order a few more and sell them on eBay or something. If there's interest.