I think you guys misinterpret his problem. I think what he actually meant was not about stability/comfort in cornering. His main problem is the bumpiness felt when driving the car alone or with his GF.. hehe. 
A lot of people actually complained about avanza being bumpy.. well... I agree as well that there isn't really a real solution to it.. you need weight to counter the bumpy..

here's a cheaper way to solve your problem.. mayb you load your mom's old washing machine in the back of the avanza.. then  go for a spin.. should cut down some of the bumpy issue? lol

do remember to fasten the washing machine to the car though.. would'nt want the washing machine to be a pinball inside the car when you bump..