You've got a nice area to work with there. Looking at coverage of major races and pics of other clubs tracks should give you plenty of inspiration.
My personal rules when laying out a track are:
1. Don't have a hairpin at the end of the main straight (causes loads of crashes)
2. Keep the lanes wider than 6ft, and make the track wider in the fast sections.
3. Have a mix of corners, and keep a flow going. Tracks that are full of tight 180s or consist solely of fast sweepers aren't much fun.
4. Place the main straight at the far end of the track. It is hard to follow your car at speed it if is right in front of you (which causes more crashes)