download and take a look at my coaster MARAUDER it has 100% custom supports it will help you see how i did it and it may give you ideas on what to do with your custom supports for your coasters. (
http://www.coastercrazy.com/track_exchang ... p?tid=2892)
basically i try to put a support vertex about 1 full grid length apart. this seems to give the coaster enough supports. i place a prefab support to see how it would look at the vertex and then design my own custom support that i think would be better. the biggest problem with custom support is making sure they do not come too close to the surrounding track while still making the support structure look realistic. it may take a few attempts to get it right. after you get better at custom supports you should not need to use the prefabs. i hope this helps. good luck.