Grab a nice thin and large book to place it under your mouse pad, that or get a piece of flat folder and tape it on top of the table to keep in place before placing your mouse pad over.
If you do not have access to anything in particular to place over it, just use paper. Preferably of a darker color to not scatter the laser.
A book, a Notebook, maps, briefcase (preferible one made of leather) any flat leather thing, cut boards