The shot cups are the best idea if thye still make them. If not, use a heavy waxed cardboard overpowder wad or cut down .410 wads pressed down on the powder, fill the rest of the case with the shot load and then place a thin overshot wad with a heavy crimp. A light target load of fast powder should be OK. Problem is not too much pressure, but in getting enough pressure, since the shot does not engrave into the barrel and blows out almost before it can build up any velocity. Another problem is making sure the overpowder wad is thick enough to keep the pressure from blowing through it and into the shot column. (I never give reloading data out publicly; too many problems if somebody goofs.)
FYI note: .22 shot shells are used by makers of new "antique" furniture. The #12 shot makes perfect phoney "worm holes."