Cokin has an IR filter, the P007. It comes as a square plastic filter and would require Cokin's adapter to be mounted.
However, the adapter would cause light leaks.
This mod shows how the Cokin filter can be cut and sanded into a circle and fitted into a screw-in filter ring.
Note: For the Cokin P series, the largest filter ring size that it can be fitted to is 77mm.
Step 1: Main material & tools required
For this mod, I'm using an old 52mm cross screen filter.
2. Cokin P series filter
For this mod, I'm using P007 IR filter.
3. Masking Tape
It'll be used to protect the Cokin filter during the cutting/sanding.
I'm using this Hunter brand. It's quite good as after removal it does not leave any adhesive residues.
This is a high speed rotary tool. It'll be used for cutting and sanding the Cokin filter.
It's possible to use a saw and sandpaper, but it'll take much longer time.
5. Safety goggles and mask.
This is a must as you have to protect yourself from the plastic dust during sanding.
6. Small flat tip screw driver.
7. Water-proof sandpaper