Cute as a button, these button-head hex screws are what they suggest for putting together a project with Adafruit's slotted extruded aluminum. Use a 2.5mm hex wrench to attach/detach. This length is good for attaching something about 4mm/0.15" thick to 20x20 extrusion.
Comes in a pack of 50. M4 0.7mm pitch 10mm threaded body.
Button Hex Machine Screw - M4 thread - 10mm long - pack of 50 (6:35)