What little weight a skid plate will make shouldn't bother. I've built more Aero Stars then you can shake a stick at and if you pay attention and put all your extras ( battery, rec. etc. ) as far forward as possible ( battery all the way up front under the fuel tank ) you should be right on, even with todays 46's instead of the old 40's.

I did notice one thing in your picture of the engine mount. Your engine is all the way back against the firewall. If you are having to add weight to the tail you might want to think about moving your engine forward just a smig. That will keep you from having to add much weight if any at all.

As stated above, if you put a spinner on that will also help with your balance. One last thing that might be of help, if it still doesn't balance to your liking you might try larger tires, especially if you are flying off grass.

It looks like you did a good job and I'm sure you will like the Aero Star, it a fine trainer, and I'm sure glad you have an instructor to help you.
ENJOY !!! RED