Welcome to the frustrating world of European plate cameras! ^__^
Those Kodak plate holders are probably for the Kodak Recomar 18 camera. Their light trap consisted of a track along the sides and fuzzy velvet strips at the top and the bottom.
Your Ideal uses a metal lip all around the opening of the plate holder to provide a mechanical light trap. That also requires the tip-in mounting since it won't slide on the back.
I have a copy of the 1931 Zeiss Ikon catalog. It lists the following holders for your 6.5x9cm camera:
Plate Holder: 726/3 -- this would require a "septum" or insert to hold cut film or paper.
Film pack holder: 728/2 -- this could be adapted for use with cut film or paper, but was designed to be used with the long discontinued film packs.
If you want to use your Kodak holders, you could build up a velvet light trap around the four edges of the holder. It needs to fill the gap between the edges of the holder and the film opening. If you start gluing things, I'd recommend gluing to the holder rather than the camera. Fuzzy velvet against a flat surface in the camera should provide an adequate light trap if the strip is wide enough.