"Are there ALTERNATIVES to using central neutral density filters?"

CND filters are expensive, even when bought used.  They can actually be worth more than the lens that it is used on!  So, of course, photographers who want them or need them want to know if there are less expensive alternatives.  And depending on you, there might be. Here are some:

Digital Image Manipulation:

Whether you are using a film-based camera or a digital one, you can use any of numerous photographic manipulation software programs to adjust the base exposure of an image and lighten (or darken) the edges and corners using, for example, a conical image mask.  This will not create parts of the image that are not there, but it will lighten dark areas, and make them appear more even-toned and natural.  And by increasing the contrast in the mask, the limited image will be more pronounced.  

Analog Image Manipulation:

If you are using a film-based camera, you can do the same sort of thing under the enlarger with dodging and burning.  Many companies made a wide variety of these "under the lens" tools to allow lightening or darkening selected portion of a print during its exposure.  You simply expose normally, and dodge the paper edges to lighten them with a negative.  Likewise, you can increase the density by exposing the film for the edges and dodge the center -- which will decrease density in the center.

Spiratone made a very handy "Vignetter" which has adjustable blades around the edges allowing them to be placed in any position to lighten up the edges of prints suffering from light fall-off.  The feathered edges assure a smooth transition.

Another "trick" is to use your enlarging lens -- wide open.  Most enlarging lenses "suffer" from some light fall-off when they are used wide open -- just like camera lenses.  So if the edges and corners of your negative are lacking in density -- and appear too dark in a print -- just use your enlarging lens wide open.  This will diminish the amount of light hitting the print edges -- producing slightly lighter edges!!!