You could tap the nut out and use a cut piece of wood veneer underneath it - and don't use any glue to start with as it's a trial and you'll probably want to change it at some stage soon. The string tension will hold it in place.
However, if the original nut's been put in with a lot of glue then there's a chance of pulling a lump out of the edge of the finger board. What I tend to do is turn the bass on it's front, and place the edge of a 1/8" chisel exactly on the join of the nut and fingerboard and then give it a light tap or two with a small hammer to try and move it vertically upwards. This tells you how well it's stuck. If it doesn't come out with a light tap then I cut it out, which isn't an option for you.
Go the 'paper under the strings' route to start with, just to check to see if the nut is too low.