Since you don't have adjustable shocks or a rear sway bar, I would imagine your best bet is to back off on the rear tire pressure. In doing this, you effectively un-stiffen (not sure of the proper word here) the rear end, helping to decrease oversteer or increase oversteer. It might take some time and fooling around with tire pressures to find the proper balance.

With my car, I have adjustable shocks but no rear sway bar. The car is neutral but at the limit will understeer. I'm hoping the rear sway should eliminate that, and also get rid of some unwanted body roll.