I’ve made modifications for a bolt-on top wing. There is a little platform that goes at the top of the cabane struts. Nylon bolts go through the top wing, through threaded holes in the platform. You can see this platform in the photo below, and if you look carefully you can see the bolts coming through the triangle stock under the platform.
The plan shows a hatch behind the firewall. The curved top portion of the firewall sticks up by itself, forming the outline of the fuselage which is flush with the hatch behind it. As you can see in my photos, I built my plane with no hatch. The top of the firewall has a smaller radius and is covered with the balsa sheet which forms the top deck. You can have your plane with or without the hatch. Please specify which way you want me to cut the parts.
I do not like balsa spars, so I cut the ribs for 1/4 x 1/4 spruce or basswood spars. If you prefer big fat balsa spars please let me know and I will cut the ribs with 1/4 x 1/2 notches as shown on the plan.