If you want a real deal magnifier, you are going to have to spend some major dollars. If you have that little rail space, a micro will probably be ideal. Find a decent optic (Bushnell Micro, Aimpoint T-1 or H-1, Eotech XPS, etc.) then buy the magnifier. Based on their other products that I have experience with, Aimpoint and Eotech would probably be the way to go in terms of magnifiers. For some of the Aimpoints, you are going to have to buy a mount for both the magnifier and the red dot (going to cost more)
My personal choice (if I had the money) would be an Aimpoint T-1 with an Eotech G33. The Eotech magnifiers have an included flip mount and are overall less expensive.
Unfortunately, you might want to contact someone with more experience than myself. I have never owned a magnifier and am basing my statements of what I've read. Just thought I'd throw in my 2 cents.
I hope this helped. =)