The bandwidth of the rugate, as for quarterwave stacks, is mostly set by the refractive indices contrast (nH/nL) in the design; the largest the contrast, the largest the reflection band. If you need sharp edges to the reflection band, the thickness required for doing that will be larger for rugate filters than for equivalent quarterwave stack, and even more if you use an apodization envelop with your rugate design.

If you need to extend the band further, then you probably need to design a filter that will have 2 or more adjacent reflectance bands on the wavelength scale.  We have published a paper on that topic (see attachment), and I am sure that you could find other similar papers.  The resulting design profile will seem complex, but not necessarily complicated to make (some of these designs are not too sensitive to errors I believe).

Article Design and Plasma Deposition of Dispersion-Corrected Multiba...