I've flown my Ultra Stick 60 with a side-mounted Saito FA-100 for almost 2 years and have never seen the need to laterally balance the plane to offset the side-mounted engine. Nor, have I seen the need to do that with any of my side-mounted airplanes. Granted, I do not pay real close attention to the finer points in my airplanes and my flying style does not suffer from it. I'm not the kind of flyer that needs to choreograph my flying and very rarely do I need to fly with my hands off the TX -- I make corrections in my flight as needed.

I'm sure you can go to the extremes in setting up your airplane and some airplanes and flying styles require it but, IMHO, this particular airplane does not need it. For what this airplane is designed for you don't need to balance it to perfection.

Having said that, if I was to laterally balance my airplane and was concerned about the esthetics I would open up the covering between the last and 2nd-to-last rib on the bottom of the wing and add the weight to the inside of the last rib. If you do this make sure you recover the entire gap between the ribs -- a patch on a transparent covering will definitely show up.