A Gridfinity 2x3 base tray, with handles screwed to the sides. This can be used as standalone or stacked with ½ unit offset on top of stackable bins. 

I use them to stack double 8u height bins in the deeper drawers of my Ikea Alex units.

Hardware needed:

• 4x M3 countersunk screws (minimum length 16mm)
• 4x M3 nuts (preferably nyloc)
• Optional: 6x2 mm neodymium magnets

UPDATE: There are now two types of handles, the original that don't stack, and a new version that allows for stacking trays. At most 8u high bins can be used if you want to be able to stack the trays.

If you want larger base trays, please ask (or modify the model yourself, STEP files are available).