There are several places on the web that describe eyepiece designs. Also, the ATM bookset by Ingalls (available from Willmann-Bell) has some good info on making EPs.
I'll note that simple designs (Ramsden, Ortho, Kellner and Plossl and so forth) aren't that hard to do but more compex designs can be done only with custom lens design and construction.
Some, like Rini and so forth have successfully done some designs, including some Erfle designs, and have sold them.

• Thanks Bob - gregted - February 17, 2003 - 12:08 UTC