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Well, you need some sort of wad over the powder to keep it in place and form a gas seal. You could use a hard cardboard wad or a cork disk as long as it's a snug fit.
Over that, you can use a spacer or filler wad if you choose, but it isn't necessary.
Powder needs compression and resistance to burn properly in a shotgun shell, so you have to have some kind of resistance to take the place of the shot. You could use rice, cornmeal or (as I remember only too well while stealing watermelons) rock salt.
Over that, seat a final thin card wad and then crimp it.
Warning! loads like this are dangerous. The filler you use can travel many yards and injure or even kill living things. If the load is too light or the powder doesn't burn correctly, you can leav a wad in the barrel. Shooting again with a stuck wad is guaranteed to burst the barrel or at least bulge it into uselessness.
If I can't talk you out of trying it (is this a July 4th idea?), at least examine the bore after EVERY shot to make sure nothing is stuck. Oh, and blanks or not, it is still illegal to discharge a firearm within most city limits. If gun go bang, you broke law. Go jail, pay fine, loose gun. (a little red man lingo there, as Rush would say, LOL!)
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