It depends on whether the access is large enough to accommodate a useful pullout or other contraption. In my configuration, a custom insert versus what it would give me wasn't worth the the several hundred dollar expense. What I did was purchase two 20 inch lazy susans for the shelves. They offer fairly easy access for seldom used things so the space isn't totally wasted.

We have the Hafele magic corner II and really like it. No wasted space, but this and similar kinds of specialty pull outs require specific cabinet dimensions.