I re-use the top card from another pack of 600 film to slide fresh packs into the SX-70.

Why the card at all? Because modern 600 film's got these 4 nubs at the pack of the pack that the SX-70 can't deal with. So you slide the card in, slide the pack over that, yank the card, push the pack in, et voila!

Update: this is how I put my ND filters on my film packs:

Loading more stuff…

Hmm…it looks like things are taking a while to load. Try again?

Loading videos…