this can cause the slot to warp...
so id go with the index cards unless u had a really thin plastic spacer.
Those are a bit thicker than would be ideal to use, I'll give you that.
My issue is with covering something like that completely with an index card or wrap or whatever. All in all, that space still can dissipate some heat. If you put an index card there, you're basically providing some layer of insulation. It may not be much, but will it be too much to the point that the card becomes a few degrees warmer?
I've bought nylon washers at lowes/home depot/menards that would provide adequate separation and yet still leave the space open. Use a spot of hot glue to hold them in place on the heatsink, and it should all be fine.