Sorry I can't help you on this one. The only bit of advice I can offer is F3A offers a great firewall mount that's fitted for the Caelus rear mounted motors, specifically the Himax.
If your in possession of the Hacker Q80 14xs I would imagine you could make it fit. If your pondering about what motor to use the Himax 210 is a great choice at a significant savings.
Big_G might be able to answer your question better, he's built three. I built one last year and flew it throughout the season and will continue to do so again this year. I've just started building another one for a friend of mine that I started last night.