Adrian,
Your twin idea would be a fun project. Read a few posts on here about twin setup since the thrust angle is so critical. With your success on the trainer, it shouldn't too difficult. Be sure and post some photos when you get her done.
It's been so long since I've designed my own that my interest now is to find older classic kit built giant planes and bring them back to as new flying condition. Each pilot finds their own corner of interest in this hobby.