Sinar IR Blocking Bag Bellows

[Marizu]

I shot some IR film at weekend but when I developed the ones exposed using my bag bellows, there was darkening around the edges that would have had most sun exposure. I believe that my (synthetic looking) bellows are not IR light tight. They are light tight for regular film, though.
How do I find an IR light tight bag bellows?
Are leather ones IR tight?

Thanks in advance.

[Marizu]

If it makes any difference, these bag bellows are for 5x4.

[Joanna Carter]

Most black materials tend to absorb and transmit IR rather well. Can I suggest draping a lightweight white cloth loosely over the bellows?