I've been working on improving the solenoid gimbal, and I've made some exciting improvements. I moved the spring to be behind the engine instead of being surrounding the engine. This means I can use a smaller, lighter spring, and it also means there is more space around the engine. This will allow me to use 24mm engines, opening up the possibility of using D, E, and even some F engines. I'm still looking to see if I can use a 29mm G size engine. This would increase the total possible impulse of the rocket by a factor of 16. Definitely significant.

Here is a picture of the improved mechanism, stripped down to the functional parts

Note that the only 2 of the 4 solenoids are in place. This is so that the spring is visible. In practice, there will be a solenoid there.

And with a live engine: (Note that the conical part on top is not part of the mechanism. It's just a mounting piece for conducting engine tests)

And a video:

It seems that the thrust of the engine caused it to stick to one side until the delay started to burn. I'm thinking that if I increase the tension in the spring, there increased restorative force provided by the spring will overcome this. I just gotta make sure it's not too much that the solenoids can't overcome it.

Overall, I'm happy with this revision. This is already enough clearance for a 24mm motor. Next step is gonna be to throw this is some sort of testing rig and see how the gyro and stabilization program work with it.