Hi Jukka,
I am sure you will get that balance point right. Just a small point on the sound.

I think at 500g it may be a little light to be really effective on a plane which will fly some distance away from you. Unless you have found a really efficient speaker, all the tests that I have done indicate that the sound output of the sound system is almost proportional to the weight carried (basically the loudspeaker weight). You seem to have the one pound sound which will give out about 100 dBa at one metre - if the speaker is not restricted. By fitting the speaker inside then you may get less than this.

The two pound sound (as fitted to the Hornet) gives out 103dBa - twice the 'one pound sound'. So if you want some more weight up front then could you possibly fit extra speakers? Although you may have tested the sound system inside and think that it is loud enough, you will be surprised how quickly the sound reduces with distance when flying.

A chap over here has fitted 'four pound sound' to his Lancaster and it sounds really good!

All the best,

John