You have to add weight as far forward as possible is key.. Friends of mine that flew in the Striking Back shows with Zeros ALWAYS had to add weight to make them balance. 4 pounds was the norm if you had the motor Byron suggested ! With that 4 pounds of lead in them they flew like a dream...

One of the ways my friends would move the weight out as far as possible is to put BIG all thread bolts thru the firewall, the Byron planes didn't have a traditional firewall usually but they did a variation on the same technique. They would run a couple 3/8 or bigger bolts thru the firewall, with a locktite nut in front and behind the firewall. They would make this bolt go up as far as they could.. Then they would balance the plane to see how much lead up on the end of the bolts it would take. Most would make lead bars and slide them on the bolts and use nuts to hold the weights in place. Often you can have the lead right behind the front of the cowl, or even partially in the front cowl lip.

I once built a Guillows rubber band ME 109 type model that was 18 inches in the kit, I enlarged the plans and built it to 80 inches. When it came time to balance it the entire plane weighed 9 pounds.. However I had to add THREE pounds of lead to balance it! 1/4 of the planes weight was lead ! Flies like a trainer.....