How can I mount a keyboard tray under my hollow desk?
There is usually some structure around the perimeter of hollow panels.  You could either:


span the distance with 3/4 material (1x3 or 4) front to back, where you could glue and screw.  You can then install the tray to the "runners".  You could miter a third piece to make it look a bit more attractive (end-grain-haters)
Span the underside with a piece of plywood glued up. Even 1/2" ply has a lot of screw holding power.  Attach tray as above.