I purchase the following two products:
iStarUSA D-214-MATX
iStarUSA TC-RAIL-20

Their product pages said they would work together... they don't. Well, they technically work together, but they don't stay within the bounds of the rack units.

The company wouldn't respond, but given that this was about the cheapest combination of case+rails on the market, I figured I'd try to make it work. With some tweaking, this 3d printed spacer go it aligned well.

Note that I designed it with relatively tight tolerances because I didn't want much play. You might need to tweak or sand it some, depending on your printer/filament/rail combination. As such, I have included the Fusion 360 project file.

Printed in ABS at 100% infill. Should work in PLA too.