Well, it the Radical Arms is too large - it won't accept a stock. So, that's exactly what seems to be the goal, a buffer tube that won't take a stock. Which is neither necessary or illegal.
I bought the PSA tube because it wouldn't take a stock (my call) and it had cool looking flutes on it. I didn't plan to put any brace on it covering them up.
As for the carbine tube that had the fin ground off a neoprene cover on it for a cheek friendly rest would be a great answer. Many use paracord. As long as the charging handle doesn't catch on it and it's secure, use whatever.
There is at present no legal interpretation or case law to indicate that having a buffer tube in military/commercial spec diameter capable of accepting a standard stock is illegal. The issue comes up when you have a pistol and stock in your possession. Like, at the range where parts are lying all over the bench, and one or two configurations are illegal. An LEO has to be there, see it, and then want to make something out of it.
For the most part we just keep quiet about what we have and the ATF keeps focusing on more stupid guys who like getting the ATF's attention and wishing them good luck in Hawaiian sign language.