I wouldn't say most kit manufacturers have things all worked out.

When I built my Tracer I asked a few questions here and found that most of them come out nose heavy. In the stock config I would have needed to add 5 ounces in the tail - which would have pushed to total weight over 6 lb, which I found awful. And it didnt' have a heavy engine, just a 46fx.

The GP UCD46 was the opposite. They come out tail heavy - there are long threads on the subject. If you used a 46 2-stroke, and the CG that Great Planes originally suggested, you'd find yourself adding 8 ounces (or more!) of lead to the nose. Re-arrange a few things to avoid the ballast and you get a much lighter plane. GP have since posted a tech notice suggesting a CG further aft which will alleviate the problem to a certain extent.

I have an LT-40 where I am not too concerned about the total weight. In that case, it has a hatch made from the rear most die (err laser) cut out. I'll add what ever is necessary when it is all together. Same goes for a Cub that is in progress.