I Use the IKEA adjustable legs (you can order them online - they are $15) and screwed them into a solid wood door (these are like hollow core, but not, well, hollow).
Hollow core doors are fine but they have a wood frame that runs the perimeter of the door, so they are hollow in the middle which means that you'd be screwing the legs into nothing. Because of that, you'd have to add some 1x6s to the door so the legs screw into wood. Home Depot or Lowes will cut them for you - just cut them to the appropriate width of the door (28", 32" or 36"), then glue - use wood glue - them to the door at both ends and in the middle; you can then screw the legs into the 1x6. Word of caution, get flat wood. Much of it is bowed. This is actually quite easy to do.
OR...use the Closetmaid Cubes (from Target)
http://www.closetmaid.com/Look/Product_Catalog/product.cfm?product_id=1976 and put a piece of wood on top of it. These are almost 36" tall.
Good luck!