I have had several Ar's over the years and have purchased 3-4 sets of the cheap ($15.00) handguards for them. To date I have yet to get a set that I felt fit properly. Most of the ones I have tried appear to be too thick in the section that fits under the Delta Ring thus they will not allow the ring to come forward to form a smooth, tight, nice appearing fit. I have even tried to thin the handguards in this area, but still the fit is poor. I have three uppers an Armalite, M&A and DPMS and it's the same story with all three of them.
If you find a solution for this please post it.
I have been toying with the idea of getting a set of M4 handguards for my pre-ban flattop, but I really don't want to spend the money for them unless they fit better than the one's I've tried in the past.