Summary

This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/exGWOjzG2BT

We have arrived at a usable version!
Virtual assembly went well, as did my initial printed version.

Printable Parts:
4x bottom
4x middle
4x top
3x Tray1
3x Tray2

Unprintable Parts:
7x M3x10 Socket cap screw per tray.

Recommended:
M3 screw tap.

Print Settings

Rafts:

No

                                            <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting resolution"><strong>Resolution: </strong>                     <div><p>.3</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>50</p></div></p>                                                        <br>                        <p class="detail-setting notes"><strong>Notes: </strong>                    </p><div><p>50% infill might be overkill, but i wanted the tray parts to be rather rigid. </p></div>

Post-Printing

Assembly

            <div><p>Use a screw (or tap) and run through all the screw holes before trying to assemble.  This will make the screws much easier to install later.<br>Take 1 pair of Tray 1 and Tray 2.  Pre-thread each of the three mounting tabs on Tray 1 with a screw. Sit the tray parts together with the mounting tabs facing one another and screw them together.  Repeat for remaining trays.  See assembled image for assistance. </p><p>After assembling the trays attach flexatx_bottom_mount to the 4 corners of the tray as seen in the assembly image.  This fit is generally tight and may require slight trimming of the mount.  When the screw is installed it may protrude as much as 2mm, a rubber standoff might be desirable or you can simply back the screw out until it does not protrude from the bottom of the mount. (I may revise the mount to correct this at a later date.)</p><p>I'm not great at writing directions, so i'll just stop here. If you reference the assembly image you can probably figure it out. </p></div>

How I Designed This

I took the ATX design specs from here: http://www.formfactors.org/developer/specs/FlexATXaddn1_0.pdf

The drive hole spacing from here:
https://www.hgst.com/sites/default/files/resources/dk23xx_ig.pdf

Used this for the switch/led's:
http://amzn.com/B00LGKQ1SS

And everything else was more or less made up.

FlexATX Tray w/2 drive mount.