We are using a 16X4 wide prop. You can use covering to close those gaps between control surfaces. Those gaps need to be as close as possible while you're building.

FYI: We used the provided engine mounts and mounted per the enclosed instructions and everything is really good. Our instructor who took it up for the first flight said this was the first plane he didn't have to put one click of trim in any direction. I guess we were really lucky.