You can send an anonymous donation. If you don't support the NRA, or at least some organized pro-RKBA effort, don't complain when "only outlaws" have guns.
However, I do not agree that there is any moral elevation that the NRA inherently lends to RKBA, any more than the Pubs inherently elevate conservativism. They may help, indeed they may do it harm (when NRA boardmember's relative got busted for armed assault giving leftists crystalized demonization fodder, for example), but being on their membership list is not a Patriotism Checklist Item for me.