Make sure the battery is as far forward as you can get it. That's the "big gun" as far as weight goes.

You might get a chunk of lead, maybe tire weights, add to the front to see how much weight it takes to get balance. It might not take a lot.

If there's room in the canopy you might be able to extend the nose a bit... bolt on an aluminum or wood plate to extend the nose and then move the battery farther forward by setting it on that plate.

Being slightly tail heavy with no fuel, it will be a bit more tail heavy with fuel.

There is little to do at the tail to lighten it. Can't very well shorten the tail tube. Guess I'm out of ideas.