THat's odd - most (all?) of the old AR articles I recall provided for a hinged cover of some sort, to avoid the problem you have encountered.

A flash thought - are you fighting the cut-off as well as the spring? You could even remove it entirely. Modern sporting rifles don't use one.

Also, could the spring be lightened by thinning it? It no longer has to do double-duty in keepeing the door closed.

What about handloading? I'd suspect the manufacturers all work to the same OAL.