you need to take one apart, and see if the 33cal balls will nest in the wad. Sometimes they do, sometimes they don't. 0.33 x 2 - 0.66"
if they don't you may not be able to get many in, and you'll have significant deformation on firing, causing a wide pattern.
if you can get two to nest, then nest the next two at 90* and keep going till you have your load. if the load is too short, add card wads under them to build them up to where you need them.
once you have determined that, then consider blown wads... you will probably have to put some hard card wads under the shot to keep the wad intact. This may be a trial and error type of deal.
the diameter of the card wads is determined by the plastic wads.
I would not try this with 3" magnum loads, but for lighter field loads, I wouldn't hesitate.
remember, make them fit is more important than maximizing the load. You may only get 8 balls in, and that is just under one ounce.
also consider getting inert filler to close the space around the balls. It is said to improve the patterns.