The two chassis are from eBay, but I machined the front panels to my design, and cut ventilation slots in the top and bottom panels
Here's a chassis before machining. Very good quality for the money.
Here's an ebay item for one of these chassis (note, this ebay link will eventually die...) http://www.ebay.com/itm/Aluminum-chassis-Power-amplifier-Enclosure-DAC-case-preamp-PSU-Box-DIY-AUDIO-/130953831714
You could post to the AMB Beta 22 Forum, they can better advise you as to what would be best with IEMs. I never use IEMs, they hurt my ears, I can't wear them, so I have not explored using any of my headphone amps with IEMs.