MVS Volvo Forums member Beck shows us how he made a chassis brace for his 2004 V70:
Made a chassis brace for the wagon from some easily sourced Home Depot items.
Chassis brace is really stiff and aside from improving the handling characteristics of the car it has also removed all the creaking you hear when pulling out of driveways etc.
Only downside of this project is that the floor boards no longer sit flush. For me, it was an easy tradeoff for the increased stiffness and sharper handling.