I have my scanner on the floor under my desk,and i generally find the weight of a foot will suffice,78's do have a bit of play,say 1 or 2 millimeters,and as they can only flex this much if you do it this way,it's pretty safe.I've added over 300 78's doing it this way,and haven't broken one (yet:).
PS. When i say the "the weight of a foot",i do of course mean your own foot:),(no trips to a graveyards with a shovel;)