Mums Bone Marrow are replacement Boros for your Billet Box. They come in a wide variety of colors and these are the only Boros I use. There are solid white and black Delrin versions, a polycarbonate clear version, and matte and polished versions in several colors.
Besides being able to get a Boro to go with the Billet Box’s color scheme, a huge advantage that the Mums Boros have is that there is no need for a condensation plug.
The prices range from $45 to $76, most of them are $60 straight from Mums.
You can grab Mums Boros from their website when they are in stock.