I have tried everything with outside the cage litter box training (Ferret)?

I have recently become the happy owner of a mischievous Fert. See the problem is he WILL use his in the cage litter box RELIGIOUSLY. The one I use outside and inside is identical, however when let outside the cage he will use mutilpe corners randomly and when he goes to use one with the litter tray in it he will poop all around it, but not in it. HE WILL POOP ON THE EDGE OF IT FOR CHRISTS SAKE! I am at my wits end, I don't want to let him out of his cage anymore because he is ruining the carpet. It constantly smells like his feces and urine. I have been to 3 different pet stores and tried many many suggestions. A list of things. I tried switching the inside and outside litter boxes, reducing his outside space to where there was enough space to lie down and the litter box, sprays of many varieties, positive and negative reinforcement the entire time, lying sleep blankets in corners I don't want him to pee in. All the time I cannot get him to use it. If I pick him up and put him in it he will get out and run about for about 5 seconds then resume his bussiness in a different corner, and if I put him in the cage, he will hold it until I let him out most of the time and try to go on the carpet again. HELP!