None of the "reversing" prisms available magnify as well, or give you as bright an image, as the waist level finder. However, I often find the magnified image of the WLF to be deceiving, as things that I thought were dominant through the finder were not so on the actual image. A 45 degree prism is really your best choice, and while it may look unmagnified, it represents the perspective of elements that will look the same as what you see on your film or digital file.
I didn't think I would like seeing the surrounding areas when I first installed my CFV focusing screen, but I got used to it very quickly, and now appreciate being able to see "past" what the lens sees.
I found a focusing magnifier for my PME51 prism on Ebay, and got it for about $45. It attaches to the viewer, and highly magnifies a small portion of the screen for critical focusing. Once critical focusing is finished, it simply flips out of the way.
If you can live with the reversed image, a Rodenstock 4x magnifier fits perfectly inside the WLF, and provides a very large, bright area for critical focusing. I've often used this setup, as it provides you with the best and brightest image for focusing, albeit "backwards."