Rusty Barrels, I feel your pain.
My 2.25 inch Hern mortar likes 170 grain charges for 100 yard shooting. That's about 1/3-ish of an ounce
I just fold the top of the foil cup over as tight as I can. There is no airspace on the top/bottom, but there probably is some on the sides in places. I do put in a couple baking soda soaked 1/8 inch cardboard wads on the top to make the powder column longer. It probably makes the foil bag compress better as well. If you do that, clearly mark the cardboard end with spraypaint.
I tape the charge to the ball with masking tape and let gravity help load it... it is a mortar and the 45 degree angle helps. I then tap it firmly a few times with a U-shaped ramrod. I'm sure there is some airspace on the sides. The top gets compressed pretty good.
Try a u-shaped ramrod to eliminate any issues.