You could, but you know in fittings how they have a lip inside them to stop a piece of pipe going too far in?
Well get a piece of pipe, cut it length ways in half, boil it or heat it up with a heat gun. Lay some bricks on it.. blah blah blah
What you're trying to make is some PVC sheet. Shouldn't be difficult.
Now once you've made some sheet, cut a disk with a holesaw the size of the I.D of the fitting you want to block off.
Make 4 of these. Glue and prime them together with PVC glue and primer.
Now glue and prime the outside of them and the front face around the edge of the disk that will sit against the lip in the fitting.
Once you've done that, put them in the fitting, and push them down until they rest up against the lip and wait 24hours for it to dry. That should seal it off from pressure.