I do not have a heater and am trying to block off the ducts leading into the car. Will the factory plastic passenger J pipe block off caps work for this purpose? Does anyone have any ideas or know off hand what size pipe caps might work?

https://maddogsmotorsports.com/

Hardware store - 2" black plastic pipe cap.

Top Secret.
Spray paint can top. Soft enough to use a clamp.

Block off the tubes in the longs at the firewall end with 2" pipe caps. Perfect fit and they even come with the clamp.



Block off the inlets to the cabin with MadDog's kit...

Click to view attachment

Heater block off kit

I used heat shrink caps.

https://www.mcmaster.com/heat-shrink-end-caps

Thanks all!

Yep- spray can top and you can get it in many colours!!

Soda can bottom a black RTV bead around the edge and stick it on. You might need temporary tape if holding it up.
We call them pop cans, you could use a beer can bottom and paint it black beforehand.

I've done this for years, looks good, fast, stays put, easily reversible and damn better than a hole.

Click to view attachment I made some from carbon fiber sheets.
Dremel cuts it like butter.