Not true. The stock class in KOH is full of nearly full frame based rigs with "real" cages in them. I worked with a couple of cage builders trying to find a way to built a removable or operable window bar and none of them could come up with a safe design. To worsen the situation, we have several companies selling bent angle down legs for the A pillar with a very small cross section that is bad.
When you say fits the purpose, that is precisely what I mean about a "predict the future" cage with compromises. You are going to build something that will not protect you in all situations so you have to have the ability to predict the future and hope you never encounter a situation outside what your cage can handle. Good luck, I hope it does what it should and keeps you safe.