Finally managed to trim it for indoor flight. Used thinner rubber - .075 vs the previous .085. Now it's flying much slower but still with enough power to climb.
Trimmed it to fly left circles. Since this is a reverse prop it's the same as trimming a regular plane to fly right circles - meaning that it's circling the opposite of the torque.
Part of my problem earlier was that the plane started flying straignt as the rubber wound down. Which is bad for indoor because it kept flying into the walls towards the end of the flight. Trimming it to fly the opposite of the torque eliminates this problem. As a bonus, the torque prevents the plane from banking too far into a spiral.
To trim it for left circles I added a lot of left thrust and some up elevon trim tab on the left wing and a couple of drag rudders on the left wing.
Here's a video of a flight from tonight's session: