Have seen and played archtops converted to mandocello by adding tuning keys, new nut and bridge. Old Harmonys seem to make the most cost effective conversions. Played a tipple once which is a guitar shaped 10 string(5 courses) instrument from South America. It's comparable in pitch to a mandolin, but tuned like a uke. Michael Kelly(made in Korea and China) has mandolas and octave mandolins made in the shape of the F style mandolins popular in bluegrass, only larger. Haven't played one, but there's a ton of commentary on mandolincafe.com regarding them.