What height are the holes you've cut? Mine are jumbos and before I realized how large they would grow to be, I bought a 4" Y-shaped PVC sewage pipe, sealed off the two ends (leaving only one entrance and a "blind" corner) and zip-tied it to the back wall. I've never seen the birds go in it (identical setups in two different cages) so I'm wondering if it might be because the pipe is too small, or the floor is too slick, or something else I haven't considered.
The way I've got the "Y" lashed to the wall of the cage makes a confined corner between the PVC tube and the dust bath, and that tight spot does seem to be a popular place to hang out.
I'd thought about an upturned plastic box but I haven't yet found one that was 7-8 inches tall that wouldn't take up too much real estate on the floor of the cage.