smegforbrains
Fish Addict

What you need
A cheap plastic seed/plant pot try, size of choice, from your local garden centre.
A length of old airline hose.
Aquarium silicone.
A hacksaw blade.
How to
Easily.
Very simply cut your openings with a hacksaw blade and rinse off the loose swarf.
Next, slice open your airline hose down it's length and use this to edge the cut you have just made for the openings.
Squirt a tiny blob of silicone into the tube at each end and at the corners of the opening to hold it in place.
Leave for 24 hours for the silicone to cure.
Then you have 2 options if your tray came with drainage holes in the bottom.
Either plug the holes in the tray and cover with substrate in the aquarium,
or,
Lay moss onto the tray and cover with a hair net. You can sew cotton thread through the holes and over the hair net to anchor the moss in place.
Finally, arrange in the aquarium.
I have found these make an ideal hideout for Catfish like Pictus and Pangasius.
You can even use a plastic plant pot instead and make a moss covered cave for a Betta.