This is the sort of thing I'm thinking of:
http://www.kustompcs.co.uk/acatalog/info_38007.html or
http://www.kustompcs.co.uk/acatalog/info_3972.html
I was looking into them a while back because I wanted to see if I could cram a 3.5" drive plus a slimline optical into a 5.25" bay. Mind you, they aren't cheap - depends if you value the storage capabilities or not. If you would value the hard drive space in there, great value for money, if you don't need it, suddenly it seems a bit pricey.
Depends how much you can source a 5.25" slot loader for though I guess. Hopefully there's something a bit cheaper elsewhere.