Run out of vitamins? need to secure a fan to a duct? I did, so with a little bit of thought, I decided to give PLA rivets a go.

Cut a piece of filament to length.
Heat one end, and press into surface ( I used my glass tabletop ) to form one end of the rivet.
Place into position in the items to be joined
apply heat parallel to surface of items so only rivet gets heated.
Press second face of rivet flat with larger flat metal object (5mm drift, or T handle as shown in pic) whilst applying pressure on the other face.
proceed to rivet other holes!

strong, lightweight, cheap, and quick!

Note that rivets can also be used as hinge pins, see the pink box, drilled to 3mm, then riveted at each hinge.

STL provided only because thingiverse required a non image file attached to this thing.