Thanks for the new source for parts.
However the poster above seems to have problems with the outer case and not the door itself.
Peter, You are Screwed!
But you can defer your dilema by buying a new set of batteries, a hot melt gun, and some strong heavy duty Rubber bands.
Insert new batteries and hold in place with a couple of strong rubber bands, then glue the door on with an ample amount of hot glue. Let cool, and shave off excess. The hot melt glue can be removed later by applying heat from tip of hot melt glue gun later. If you do this right, you will probably get two or three battery changes out of this using this method.
I would also suggest buying a used Hp48G and upgrading (or some other RPN, if you really want to continue using RPN in the distant future). I suggest the Hp48G as it is most similiar and yet improves upon the battery door of the 28 series. However, the 28 series is also fairly reasonable to still buy, even on ebay.