I loved @Jerrari ‘s Quicklock Drawer system.  

https://www.printables.com/model/312837-gen2-drawers-large

But I have a table that has a support in front. So it’s not flat.  So I designed this so The top rail guide can mount to the front support and this spacer is for the back support.
The one in the pictures is for an earlier design and I didn't like the way it printed. So the model is the latest one.

The model is a spacer height of 84mm.  You can load the step file in you modeling program of choice and change the height to suit your needs.

You need to place the step file on the larger, wider face, not as it comes into the design, probably

This model is designed to capture a nut mid print.  So you have to pause this print at 80.2mm and insert a M4 nut in the places you want and resume the print. If you change the height then you'll have to adjust the stop point. The nuts I used are 3.1mm tall.  I used  M4 x 12mm screws to fasten the rail guide to the kept nuts of the spacer.

I've included a single, double, triple and Quad step files.  I've only printed the triple in this format and the double in the earlier version.  So I don't know about the single and quad versions.