Dennis
What did you mess up on the 4ft machine. If you built it right it is a super little mill.
I agree with Dan about running a blank disc if you can't afford magnets.
I have just modified one of the Hugh 4 ft machines to take a 5'6" prop in a low wind area just by adding a blank rotor, but I did start out with a different mechanical arrangement as this was the intention from the start.
With different coils and a higher tsr it should manage a 6' prop with the blank rotor & the 8 magnets.
You will need 12 magnets to equal that with a single rotor, so if you are short of money it may be cheaper to buy another disc than more magnets.
As always it depends on what you have, what you can do and what you can scrounge as to which way works out cheaper, but if magnet is your main cost an un-backed single rotor is not very cost effective.
Flux