Ferrets refuse litter box

Question
I have two ferrets that were born in February 2008. I have had them for about a month now. I'm am trying to litter box train them, but they refuse to poop in the litter box. I've tried putting their towels in places where the litter box isn't, but they just poop on the towels. I have a Ferret Nation cage, so clean up is pretty easy; and I don't mind cleaning up after them...I just don't want them to get into the habit of pooping and just letting me clean it up. I'm afraid that if I can't get them to do it now, they will never learn in the future. (Also I don't want to put newspaper down where they poop because I don't feel like that is a solution.)

Currently, they have access to the whole house (fairly small) and they have lots of tubes, toys, and dig boxes to play in. I bathe them weekly and clean their cage regularly...I've tried to make it as easy as possible.

Answer
Most of the suggestions I have are not really long term although they might help.  First, add more litter boxes.  Some ferrets won't poop in bedding while others will.  Second, change litter?  Try a recycled newspaper since the litter you use may not smell right or may be more fun to play in.  Third, encourage them to go in the litter box - place them in it and after the use it, let them out to play.  Newspaper can work if they get that the newspaper is for going to the bathroom, you can slowly move it closer to the litter box then in the litter box to help with that association.  Finally, just know that some ferrets will never use the litter box consistently.