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Don't make this complicated. The washers seal the sides of the fitting. The threads require additional sealing. You can use Loctite, an acorn nut, a nylock, pipe thread sealant, etc...
If you're using a nut and bolt you're going to need to use a capped or acorn nut otherwise air can leak along the threads of the nut. A new nylock nut may also seal sufficiently well.
You can use a standards hex bolt with the sealing washer as a plug.
Or, you can get something like banjo to JIC adapter and cap it.
https://www.mcmaster.com/#4545K311