IMPORTANT:
A couple of commenters have said the drawer is too wide for their  Ender 3.
Please measure the gap between the Y axis and right hand extrusions before printing anything - if the measurement is less than 115mm (or 105mm if you have a Pro) it's likely the drawer will be too tight.
This simply appears to be poor quality control from the factory, with the Y extrusion being fitted off centre/out of square. You may be able to realign it by loosening the attaching  bolts underneath the cross bar.
Update 25/09/18:
Modified version for  Ender 3 PRO - see below.
Update 14/07/18:
Y-axis tensioner change - see below.
There a few drawer/tray type things for the  Ender 3 already on Thingiverse but none quite what I wanted.
This drawer slides between the Y axis and the right hand horizontal extrusions, it is supported on both sides. When closed the front of the drawer is level with the end of the right extrusion, when opened it is level with the end of the Y axis extrusion.
You just need to temporarily remove the…

Check out jgvonk1984's collection Creality Ender-3 Drawers: .