Models without internal formers, What to do??

Hello my friends. I have purchased some beautiful models that I want for my collection. To my surprise, these models do not include internal fuselage formers which are indispensable and mandatory for obtaining not only correct shape but also rigidity. If formers are not included, what is your advice on solving this issue? I have tried to build a former less model with no success at all. Models come out with wrong shapes and very flimsy. Also very hard to make. Murph's models and Fiddlers Green are masters in selling card airplanes with no formers thus, hindering the assembly, sometimes impossible to build. Thanks for your help!!
EDUARDO