+1 on Wazz's comments about putting the strap button on the neck plate, here's a photo of mine, in the top front corner just below the bolt - it balances perfectly.

I'll be interested to hear how you go with the thumb rest, I'm also thinking about fitting one. Resting my thumb on the bridge p/up works pretty well but I'd like the option of playing a bit further forward too.
Click image for larger version. 

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