Here’s what you’ll need:

• Substrate for the moss, such as high density foam, about 2.54 cm (1 inch) thick
• Silicone sealer, like caulk or cement
• Posterboard or cardstock for creating stencils
• Marker or grease pencil
• Moss (many kinds of moss can be used, just choose yours based on the amount of sunlight and average temperature of your bathroom)

Here’s how to do it:

• Choose the size of your mat. If using foam, be sure that you have enough to make two layers. Bamboo or wood trays will not work quite as well, given their tendency to produce mold and mildew under the conditions required to keep the moss alive.
• Choose the shape of your mat. You will need to create several cutouts for your moss. Use the marker or grease pencil to trace the shapes onto the foam and use the utility knife to cut through the entire thickness.
• Seal the mat. An adhesive like silicone caulk is recommended, because it will create a water-tight environment for the moss. Using a product like hot glue or certain other adhesives may melt the foam.
• Prep the mat. Once the mat is ready for the moss, it will need to be properly prepped to ensure its survival. [M]ist the surface of the mat.
• Plant the moss. Insert the moss into each cutout.

Depending on what species of moss you have used, you might need to water it more than just the drips from your shower once a day. Use the spray bottle to mist the moss for the first couple of months while it gets established.

A clever way to deal with the inevitable fact of damp post-shower realities and turn it into a benefit.

Commercially available moss mat pictured above from SVVPLY for $100-200.
Via IFL Science!