The map Aerocrest has inspired me to make a race track that takes place partially in a cave made in the sky bubble. It is going to be a circle with a closed and unclosed part of tunnels and grass. A lot of jump and obstacles too. I'm not very good at making maps but I'm trying to. Maybe someone can make one too.
What I want to know is if anyone has managed to put in a working loop on a racetrack. That'll be a definite DL for me. Although, it seems like creating a tunnel of any decent length in skybubble will take up a lot of resources. Maybe if you looked at the race map thrashher, which is half on the ground and half lifting it the air it might work. It'd be really sweet if you had a tunnel built along the steep dune.
I can see you might have object issues here or the map might be really short. This might be the reason why it hasn't been done yet. Make sure you plan this map carefully because you don't want to get to the end of it and find that you haven't got enough objects left to buy the mongooses.
I'm currently working on a totally enclosed racetrack in the skybubble: The Mines in case you're interested. It's not going to be the longest track in the world, it takes about 25 seconds to complete a lap, but due to the amount of budget which went into creating the walls and everything it was always going to be pretty short. The best bit of advice that I can give you is to first make the actual track, then add the cave structures and obstacles. That way even if it isn't all enclosed, at least you can go round the track. Best of luck with the map man