alfie- I did the same thing with my Sig 1/5 Cub, converted it to an L-4B, complete with the greenhouse. Although they don't make the kit any more, BUSA still has plans available for the L-4 in 1/4 scale. They show thw greenhouse, but for some reason the center section is still "skinned" over the top of the cabin, I guess to conceal the wing tubes. The plans would still be a help to convert the main structure, and they're not too expensive. My 1/5 scale also used aluminum wing tubes, but I eliminated them in favor of staying true to the scale details. I just added another bolt to each wing where they attach to the center section (makes two per wing half), plus the struts were already functional. I reinforced the angled uprights at the edge of the windshield with carbon fiber and used hardwood for all the center section pieces. The weight gain is well worth the extra strength. Here are some pix of mine to show how the center looks.

I have a 1/4 scale BUSA Cub to build later, and I intend to build it to look like my 1/5 scale. I will follow your thread to see how your build goes.