Im sure something like this has been done before. But Im thinking like Mini Orange Slalom flags with a spring on both post that can be pressed into the infield line. You can even have a couple green ones in turn four start area. Track markers only, That wouldn't upset he car yet you wouldn't want to race over em yet hopefully not get knocked out into the track if they got hit.

Personally, I like the un-runable berms that are soft maybe six inch tall marble material. If you make em sold and packed then guys just setup to put their left fronts on em and weight transfer.