I have a brand new (uncarpeted at present) staircase where the bottom riser appears to be about 1mm too short which then allows the tread to flex and emit a squeak from the joint between the two. Admittedly there's no glue there as I was left by the builders to fit the bottom riser in situ. I have a mind to get some glue in there but, ideally I'd like to put in a tiny bit of packing under the riser to stop the movement. Any ideas as to what would best achieve this? I'd thought of a thin piece of hard plastic but haven't found anything suitable.