Cut your own plug from a same depth of plywood with a hole saw bit the same size? Wood glue and sawdust to fill any gaps
Cut a plywood plug that fits inside
Fill it with epoxy
Measure the diameter of the circle and cut something into a circle that is roughly 2-3mm smaller in size and the correct height. You could use anything that is compact/dense enough that it wont collapse when applying a heavy load (i.e. cardboard wouldn’t be a good choice). You could even get the circle of the correct size 3D printed.
Fill it with water and give some nearby frogs a nice pool.