Trans Pan & Filter
I'd check with the pan manufacturer and see what they reccomend. It could use an extension tube or different filter. Like Crosley said some have 2 small stands in the bottom of the aluminum pan that keep the stock filter from dropping out. If yours doesn't have any stands it probably uses a filter with a longer neck or an extension tube. Or does your filter have a bracket that mounts to the Valvbody to hold it in place?