Toileting in bed!

So since I've moved Flo and Tubsy onto concrete (due to fox visits) they no longer toilet outside - at all!
On the grass they nearly always did it outside!
Just means now I'm cleaning their bed section out a lot. How can I encourage them to toilet outside? They have a litter tray in the run and also one of those large cardboard fruit boxes filled with hay.
Thanks!

Can you make their indoor space into a large litter tray? Then it wont matter if they toilet in there.