I'm ordering lenses from Surplus Shed for my big solar projection scope. The plan is a 150mm PNM objective lens with a 2600mm focal length, followed by a -800mm MNM lens spaced at 1890mm from it. This should result in 202mm image of the sun approximately six meters away, for an effective focal length 23.1 meters!
I still don't know how I'll direct sunlight into the scope. My first attempts will use a hard drive platter as a super-cheap mirror.