Litter Training Help

I had two Netherland Dwarfs, Smudge and Daisy who lived together for 10 years until sadly last August Smudge died. They had lived in a two up two down hutch using the dark side upstairs as their toileting area.

Following the loss of Smudge, I rescued Benny, now called Merslin, and together him and Daisy moved into a bunny playhouse which is three tier. On the ground there is a dark box in which I placed a plastic storage tub with an access hole and hoped that it would be used as a litter area however Merlin and Daisy seem to do all of their peeing and pooing on the middle shelf of the playhouse. I have dog fleece on the shelves and even though it is easy to wash it requires changing every day and it soaks right through to the shelf which, at this time of year, is not easy to deal with. I have tried putting peed on newspaper and poos into the litter box but to no avail.

I am at my wits end. Any help or advice would be appreciated.

I have some photos but not sure how to upload.

I would remove the fleece, my outside bunnies always use fleece as a toilet. I would cover the shelves and floor with non slip vinyl flooring, or rubber matting. Are they spayed/neutered? If they aren't, that would improve things a lot. Territorial marking can account for a lot of stray poops. Is the tray large enough for them both to sit in at the same time? I like the underbed storage boxes for trays as they are larger and have higher sides than cat litter trays and stops them from spraying over the edge. You may need to provide more than one tray, if the shelf is large enough I would consider putting one up there seeing as they like to use that area as a toilet,
Sweeping up poos regularly and putting them into the trays would help too, and also mopping up any wee and cleaning the area with a diluted vinegar/water solution to stop them re-marking the area helps.