You need a tray anyway, so that you don't have to carry your coffee dishes individually from the kitchen to the balcony. Just leave the dishes on it and use the tray as a table. It couldn't be easier! To be able to put the tray down, assemble two supports, best pivoting with retaining lugs, so that the balcony table can be clamped in between and locked in place. That also works, by the way, when you need a surface without a border and turn the tray over. If you don't want to drill holes in the balcony parapet, sets up fence elements wood, into which the screw eyelets for the carriers are screwed.
The tray is constructed from plywood. Tenons ensure stable corner connections of the sides, which one saws with the coping saw.
The plywood parts are assembled into a tray with glue and pins. Nail carefully, so that the tips do not drift out of the wood.
The swiveling supports for the tray come from the Fischer system (for garage equipment, available in hardware stores). They are simply hung in the associated screw eyelets.