Heres a link to one of the companys that do urethane injection. you basically drill some holes in your side sills and b pillars and pour in the mixture that hardens inside and stiffens up the chassis.
these guys sell it for 19000yen and another 50000 to 60000yen for putting it in your car, or you can DIY but if you make a mistake while putting it in the sills it is impossible to remove.
All the positive reviews ive read are from people who've used it and had success. All the negative reviews are from people that havent tried but point out all the things that could potentially go wrong with it