A Gridfinity 3x3 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: 6x2mm nedodymium 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.

I have also uploaded a 2x3 tray. If you want different base sizes, please ask (or modify the model yourself, STEP files are available).