Not sure if appropriate or if anyone is interested - if not, I apologise;
I've used a 150mm x 105mm x 55mm aluminium project box (~AU$15) to house these. With the Pi mounted front and left, the volume knob is on the centre line, and the space to the right of the Pi allows a stereo plug to connect to the headphone jack on the X400 (I run that to a 3.5mm socket on the front). There's room at the back to allow a DC jack to run to a switch and socket, and to mount speaker terminals, as well as a panel mount USB socket to break out the Pi's USB. I have toggle switches connected to the jumpers to allow the speaker and headphone amps to be switched on/off and to engage the mute function. In all, another ~AU$15 in jacks/terminals.
I've models (https://www.thingiverse.com/thing:2539748) for the front and rear panels, and a mounting plate for the Pi to suit a box such as here (http://www.aliexpress.com/item/Aluminum-Box-Enclosure-Case-desktop-DIY-55mm-2-16-H-X105mm-4-14-W-X150mm-5/32342220019.html - no affiliation, it's just what I've used).
Back, front, interior and a shot showing the Pi LEDs through the PETG front panel. The last shot is of the first of these I made where I used the project box panel and drilled and filed holes in the Aluminium (tedious). In the last shot, the Pi LEDs are visible through the 1mm hole below the 3.5mm stereo jack, but because they are off the mid line, they reflect off the polished sides of the hole rather than being viewed directly. I thought I'd have to play with some kind of light pipe, but this works well enough.
If there's anything else I can show, please let me know.
I've used a 150mm x 105mm x 55mm aluminium project box (~AU$15) to house these. With the Pi mounted front and left, the volume knob is on the centre line, and the space to the right of the Pi allows a stereo plug to connect to the headphone jack on the X400 (I run that to a 3.5mm socket on the front). There's room at the back to allow a DC jack to run to a switch and socket, and to mount speaker terminals, as well as a panel mount USB socket to break out the Pi's USB. I have toggle switches connected to the jumpers to allow the speaker and headphone amps to be switched on/off and to engage the mute function. In all, another ~AU$15 in jacks/terminals.
I've models (https://www.thingiverse.com/thing:2539748) for the front and rear panels, and a mounting plate for the Pi to suit a box such as here (http://www.aliexpress.com/item/Aluminum-Box-Enclosure-Case-desktop-DIY-55mm-2-16-H-X105mm-4-14-W-X150mm-5/32342220019.html - no affiliation, it's just what I've used).
If there's anything else I can show, please let me know.