We used to have an English Mastiff and they are big dogs. The readily available cages were not big enough. You could order some bigger one's online, but thr y seemed awfully flimsy when they got that big. I finally did find some commercially available solid, beefy ones but they wanted an arm and 2 legs for them.
I had plans to build one almost the same as what you did, but with steel because I'm poor. I guess we just realized we didn't need a cage that badly, as it would only really have been for transportation and when we did take him anywhere we got by alright, so it never got built. I understand why you built it.