This is the way I remodeled the roosting bars:
The construction sits on top of the poop board.
None of the chicken should be able to connect with the coop walls while sitting on the roosts.
This almost sounds as if your coop has insulation?
To get rid of red poultry mites I would proceed as follows:
Clean out your entire coop including nestboxes, feeders, water dispensers etc. thoroughly and burn the entire bedding immediately. Soak all the mobile stuff in vinegar essence...