In my opinion we really need to know if there is a specific gluteal muscle you are trying to isolate or just "the glut's" in general. EMG studies show that the single leg dead lift (shown here YouTube - Single leg deadlift) has the highest maximal voluntary contraction. Prone SLR is ok and I use it with people with very weak hip extensors as a form of AAROM as with a straight leg the hamstrings also assist. A better way is prone with the knee flexed. This places the hamstrings in active insufficiency minimizing there contribuiton, thus isolating the gluts. Another modification I do is to have the pt bend over a table with feet on the floor. This allows an increase of ROM extension from ~90 of hip flex to 10-20 degrees of ext (or whatever is available to the pt). (almost like a donkey kick).