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.
Modified version for Ender 3 PRO - see below.
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 control panel to slide the draw in. When refitted the panel stops the drawer from pulling right out.
Owners of later Ender 3's with the double sided tensioner may find it a little trickier to fit the drawer, however there should be enough flex in the model without removing the tensioner.
The drawer has a cut down front so you can access the bed adjuster on the fly and has a stopper on the back to allow plenty of room for the control panel cable as well as other cables (eg: SD card extension).
There 2 versions, one with a divider, one without.
Modelled in Sketchup, with 1.6mm walls and bottom, but I've included the SKP files if you wish to make them a bit stiffer or do any other changes - you can export as an STL with the free extension from the Warehouse.
Newer Ender 3's have a double sided Y-axis tensioner and the original drawer wont pull out all the way. The files section has been updated with new STL and SKP files for those models - drawer1-update & drawer2-update.
New files have been uploaded for the Ender 3 PRO which has a wider Y-axis extrusion - drawer1-pro and drawer2-pro - Sketchup files also included.