I use milk creates with one side (waffle down to soild) cut out and to secure them I cut 3/4" press board and screwed it into the creats and then covered that with floor tiles (the kind with the sticky backs on them) I had them laeft over from another job I did then I put pine shavings inside them makes for easy clean up.
If you go with buckets and shavings rememeber to cut the lid down to keep in the shavings.
Good luck to you