Good morning, Michael;
For my own SINAR F and F1 and the Graflex Speed Graphic, the ones I like the most of those here are the plastic Fidelity Deluxe 4 by 5 film holders. There are about 25 film holders here, the result of about three years accumulation, and most of them are the Fidelity holders. So far they have worked well for me. They are easy to clean and load, and they are very stable over the temperature range in which I am fairly comfortable, from about +10 degrees F. to about +110 degrees F. I have had no problems with light leaks or other problems, but I have not really pushed them in that way either.
OK. This is from a guy who has been working with photography since 1960, but got into large format (4 by 5) just 3 & 1/2 years ago. There may well be others with much greater experience than I, but you and I do have the common point of both of us being fairly new to large format.
And, welcome to APUG. That is an interesting short biography you have posted.