That sounds good!
If you're going to all that effort, it may be an idea to include fields at different alt, it'll be a long wait for climb out should there ever be a Lanc competition

With regard to a canyon walled arena, perhaps it may be nice to have "eagles nests" type fields embedded in the Canyon walls at differing alts, if that's not too fancifull. Seeing as the intention seems to be to fly one plane per evening, it might be nice to give the CM the option of having, say, a round starting at 0K, 10K and 20K through the event.
What about a Jabo terrain? That would need scorable objects, but a nice obstructed ingress path would be good (twisty canyon?), or it could quickly devolve into a lawn dart final!