One of the simplest designs is the double achromatic, essentially the same as a Ramsden (I think Ramsden is right; it's either that or Huygenian) eyepiece for a telescope. It's just two achromats of the same specification, facing opposite directions (usually with the convex or more convex face out), spaced according to formula, with the aperture stop midway between the lenses. You can likely buy suitable cemented achromats from Edmund, or you may be able to get them from resources that sell to amateur telescope makers who like to make their own eyepieces. Small telescope objectives should also work well.
One advantage of symmetric designs like this is that they have no distortion and a flat plane of focus, as a simple consequence of the optics (providing they're spaced correctly). A disadvantage is that they perform best at close to 1:1, and tend to have a small amount of uncorrected spherical aberration and some coma at common photographic magnification factors; this can be largely overcome by using a small aperture, or used to advantage by calling the resulting optic a "portrait lens".
