I was thinking of open cell rubber foam, similar to a bath sponge but denser and more rubbery. Like the head used on the Floor Whizz. https://www.verimark.co.za/brand/floorwiz/
As long as the sponge is exposed to water it will provide a huge internal surface area for denitrification to take place.
So my thought was to mould the DIY mix around this sponge, taking care to either leave some of it exposed to water (either a very thin coating over the sponge in this area, or a "false cover" which would allow water to flow underneath the cover but hide the exposed sponge area.