Canadian luthier Yanuziello makes 8-string semi-hollow electric mandolins, mandolas, octave mandolins and mandocellos. He writes: "My electric octave mandolin and electric mandocello are basically the same instrument. To change one to the other requires a nut replacement, and of course a different string gauge. The scale length is 24.75 inches, which works for both. The different string tension is no problem for the neck / truss rod."
     Full disclosure: I play one of Joe's mandolins and I absolutely love it. You can or write him:

Yanuziello Stringed Instruments
53210 Winger Road
RR#1 Wainfleet
Ontario L0S-1V0
Canada
Telephone: 416/534-0419