Try a litter tray which can be fastened to the cage, or something heavy enough for them not to move. I use a big old casserole dish. Rectangular is best, corner trays are harder to train to.
If they're digging in the litter, make sure you're providing something else for them to dig in instead, like a dig box filled with rice. If they still dig in the litter tray maybe just line it with a sheet of newspaper, or nothing. I have no lining in mine because of digging.
Have you spent time working on litter training? Place the tray in the favourite poop corner, place bedding, food and water to take up the rest of the corners. Put a fresh poop in the back corner of the litter tray. Keep the rest of the cage clean and poop free at all times. Try using lots of plushy fleecey blankets to cover all the floor, they're less likely to poop on these. When you see them backing up to poop place them in the tray. When they go in the tray reward them with treats or fuss. Stick at it, it takes time.
The litter tray needs cleaning daily, maybe twice a day. Some ferrets are fussy and won't use a dirty tray as they don't want to get their paws mucky.
As for out of cage time, training is virtually impossible.. if they have a favourite spot you can try a tray there.. personally I just put some paper / an old towel that can be washed down in the favourite poop corner of the room...
Now that they use the tray, I clean that every morning, and in the evening if it needs it, and wash the bedding and scrub the floor twice a week. It's more than sufficient and doesn't take too long.
I also noticed that about a month after switching to a raw meat and whole prey diet there was a lot less poop.. just an interesting observation, if it's something you'd be willing to consider. It has a lot of health benefits for the ferrets, but can take some work to switch them over and make sure the diet is balanced. The holistic ferret forum is a great starting point if you want to look in to it.