Generally speaking,
below has to do with vertical levels.
below is 'lower than'.
under usually includes the idea of 'covering' as well as the idea of 'lower than'.
under is 'covered by'.
I would choose under for your sentence, but below is also possible. When both are possible, choose under if you wish to emphasize that the location has something which acts as a covering or ceiling (under a blanket, under a table, under a cloud of suspicion).
CJ