Hi .I keep my birds inside too but I use a very very large box for them to live in. The box is large enough for me to lay in both length wise and width wise. I fill the box with pine shavings and they take their "dust" bath in the shavings themselves. They kick their feet out and purr loudly while doing it so I suppose they are enjoying it! They have plenty of room to move about comfortably in their box. As for a caged bird, how big is the cage? If it will fit you can put a tub of dirt inside use a large clean cat box pan to hold it. This might work, then if they wish, they have a area to dust bathe when they want. Hope this helps. Or you can try keeping them in a very large Box with shavings, Then They will bathe in the shavings. Hope this helps and gives you some ideas. Best wishes.