It seems the equivalent of a bushing die could be made easily if you can't find the sizer. Another slower option would be to buy a used original whack a mole Lee reloader the sizer is very effective and if used on a base with a hole for the primer should be safe enough. I know I have 12 gauge sizers of various types but not sure about 20 gauge.

The pipe flanger sounds like a good option.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli