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
Step 2: Remove Filter Glass From Screw-in Filter
Insert the flat tip screwdriver into the gap of the retainer spring and pry the spring out.
After separating the filter glass, retainer spring and the filter ring, keep them aside first.
Step 3: Prep Cokin Filter
This helps to prevent scratches during the cutting and sanding process.
Step 4: Trace the Shape of the Filter Glass
Step 5: Cutting Process
Wear the safety goggles and put on the mask before using the dremel.
Step 6: Sanding Process
Using a Dremel sanding band, sand away the remaining plastic of the Cokin filter away.
The glass filter element will prevent you from over-sanding.
A piece of wood can be used to prevent the plastic from vibrating during the cutting/sanding process.
You might need to use sandpaper to sand the edges down to fit it into the filter ring.
Try not to over-sand till it's smaller than the original filter glass, or else light will leak through.
Step 7: Final Step!
Wash the Cokin filter under a running tap.
Wipe it dry with optical cloth.
Do a fit-check. If it can't fit, use water-proof sand paper to sand the edges down.
Once the filter can be filter properly into the filter ring, install the retainer spring.