Most IR pass through filters are for 890-900nm not 940.
I ran into this issue myself for a slightly different use case.
The thing I found worked surprisingly well was a scrap piece of car window tint. I literally just walked into a local shop and asked for a small piece of scrap.
My circa 2016 DIY attempt at an equivalent of TrackIR used a piece of floppy disk media as IR filter for a PS3eye camera (I used 850nm LEDs tho) .
More current howto linked to filters at aliexpress, and they have all sorts of ir filters in 940nm, for things that look like the super cheap IR backup cameras. Maybe a dirt cheap backup camera housing+filter could work…
Also, that old wiimote might have new purpose. Look at step 7 here.
You want it opaque in the visible spectrum but transparent to infrared? Unexposed (but developed) Ektachrome slide film works for that. I had a piece of it around here from some years back but can’t find it now. If it turns up I can send you some. It’s from when I was fooling around with the night vision feature on an old camcorder.
I’ve used and tested the same sensor with a wjndow made of 4905 transparent vhb laminated onto clear pet film.
Are you trying specifically to filter out non-IR frequencies?
No, I’m just trying to keep it clean and easy to clean.
Then most transparent plastic will work.
To be sure, find out exactly which material is used in whatever you’re considering buying (PC, PET, acrylic, glass, etc) and look up it’s transmission spectrum. Make sure it is high at 940nm.
