I'm very happy that you maked possibility to load external maps too, thank you! I have knowledge in 3D modelling and I have cool real places near me to fly so in couple weeks new maps will come! You're man!
Please note that the custom levels will need to be created with FPV Freeriders own level editor to work.
And, I haven't released that one to the public yet.
So you will need to wait a bit, I plan to release the full level editor later on when I have evaluated it thoroughly.
It is pretty similar to Unity's own editor though, so your 3D modelling experience will come in handy.
I would keep it simple... community can make levels in Unity's own editor... then all problems are gone... I think that it will be a lot easier to make levels in Unity as I can do modelling in whatever software I want and load it to Unity's editor and then save them as Unity's and simply load them in FPV Freerider... The editor is there for us all... and it's free... no need to complicate.