In terms of flights you can get to Frankfurt only from Edinburgh. Ryanair fly, as you'd expect, to an airport pretty far from the city. It's a long time since I was in Frankfurt and before my punting days, but I remember it being pretty far out. The only other option is Lufthansa to Frankfurt airport. That might be a bit more expensive at this short notice though. It's worth a look I guess.
You might be able to fly to Dusseldorf from Glasgow with Germanwings (Lufthansa's budget brand), but that's even further away from Ryanair's option. There are options in Dusseldorf though...
I had my first FKK trip last year and will be making one or two this year. Frankfurt is definitely on the hit list

. You'll be addicted in now time!