Sand the entire furniture once more to make sure there are no rough edges and paint it to your desired finish.
For the seat, cut 3/4 plywood to 42×44 cm and round off the corners.
Glue the foam mattress with super glue or any glue you have to hold them together.
Then use staple wire to tightly cover it with your chosen fabric. The one in the picture is fake leather.
Secure the seat in place by screwing at the corners it from below. Therefore you have to make sure you have four screws that are long enough to go through the base board and hold the seat, but not too long that it also goes all the way through the plywood. You don’t want to get pierced by the screws as you sit! :)
Thank you vatkin for the great idea!