If you are shooting clays or hunting it is recommended to use as thin & light an over-shot card as possible, so that it won't disrupt the shot pattern. These need to be crimped or glued inplace, especially if fired in doubleguns or magazineguns.
However, if you are shooting CAS you can use the GRAF 14ga. nitro card on top of the shot also. I think this will be large enough to stay put without any crimp or glue. You might have to enter it into the case-mouth standing on end, and then spin it around inside the case and seat it.
For comparison: I load MagTech 12ga. brass shells for CAS, and use 11ga cards and wads down in the shell, but on top of the shot I use a 10ga. nitro card from Track-of-the-Wolf, and it stays put just from friction fit.