I picked up a Vision Engineering Mantis Stereo Microscope a couple months ago. I do a lot of soldering of really small components, so this is really nice for soldering and inspection. The Mantis is really cool with a large open face, rather than small eyepieces, but still has two optical paths for stereo vision (and you can move your head a bit to look around). I got a great deal on it, but it came with no lenses... and even used chipped original lenses sell for ~$150, so I decided to make my own, using lenses from cheap jeweler's loupes I had laying around.
These jeweler's loupes have 1" glass lenses, and sell on ebay for ~$6 shipped for a set of 4. Of course, you could use higher quality lenses if you have some, but the quality is actually pretty good from these.
I don't know whether the loupes are exaggerating their magnification, if they perform differently because of the Mantis optics, or if 'x' magnification even has a standard... but I compared these lenses to the specs given in the Mantis manual for the original lenses and determined that the "2.5x" is equivalent to 2x, "5x" is equivalent to 3x, "7.5x" is equivalent to 4x, and "10x" is equivalent to 6x. The lens_specs.txt file has the working distance, field of view, and depth of field specs from the manual.
The Mantis can have two lenses mounted at once. The original lenses from 4x-10x are parfocal, meaning that you can switch between any two lenses and you won't have to move the head to refocus. In my case, I liked the 3x for normal work, and 6x for close-up work, so I used the 3x lens in the short one, and made the 6x the correct length to be parfocal with the short 3x.
I also included the ability to "stack" a lens on the 6x, so when I need REALLY close up, I can stack the 4x on the 6x and get 10x. Of course this is no longer parfocal, the quality suffers a little bit from using two lenses, and the working distance gets pretty close, but it works for the times that you really need to look up close.
For attaching to the microscope... rather than trying to screw on the original threads, I noticed that there were flat spots on the sides of the threads, so I also made the adapter round with flat spots. This let me pop the adapter straight up in place, and lock it in with a quarter turn (so the flat spot is grabbing the threads). The end of the long adapter, as well as the additional stacking adapter, are both oval shaped. This lets the adapter pop in place and lock with a quarter turn as well. The long adapter is tapered so it doesn't block light from the halogen lights at the focal point, even with the higher magnification attachment.
I'd recommend first printing the short adapter (loupe_lens_adapter_short.stl) and verify fit (much quicker than printing the long one). You may need to slightly tweak the X and Y scale to get a snug fit, depending on your microscope and printer.
Once you print the parts, you need to extract the lenses from the plastic loupe housings. On most of mine, I was able to simply press them out (push from the top out the bottom). One was glued in pretty well though, so I used wire cutters to break the plastic and pop the lens out.
The lenses may snap right in place on the printed adapters, though my lens sizes varied from 0.93" to 1.0", so some fit better than others. A small dab of glue, epoxy, etc. will hold it in place just fine.
Once the lenses are in place, simply install the adapter on your microscope and enjoy!