Thanks for the information.

I have done a fair amount of experimentation with fillers starting with reduced "gallery" loads in the 44 magnum.
At one time I was interested in finding the lowest velocity load that would be accurate and still leave the barrel.
I ended up with 4.5 gr. PB and filled the case with GREX shotgun buffer compound.
That's still one of my favorite fillers although "out of print" for years now.
It's very light, somewhat compressable, and can be metered thru a powder measure or weighed on the scale.
I also use it in my 44 magnum equivalent reduced load in the 444 Marlin.
24 gr. 2400 + 14 gr. GREX for 1740 fps with a .44 240 gr. JHP
Other materials I tried that I had laying around were case tumbling media, both corn cob and ground walnut shells.
The corn cobs worked well as it also is light and compressable.
The walnut shells are much denser but meter very nicely.
Upon being toasted they produce a smell very like that of a certain well known recreational herb.
This caused some sharp looks at the range.
I also noticed the bores were very clean and shiny and wondered if I was over polishing them.
Gave up on the walnuts.
Also tried and abandoned:

PuffLon: This is VERY fine .. like dust and has some moly in it.
Quite the PITA to handle, measure, and dispense without making a giant mess.
The slightest breeze, like breathing anywhere near it, blows it all over and you can never really clean it all up.

Toilet paper: This is cheap and easy to use but is shredded and blown out in front of the gun leaving a big mess on the floor of indoor ranges.
Range master not happy.
Outdoors OK but the security of the powder is not great as the "seal" isn't perfect.

Yet to try:
Foam earplugs. The ones I have are open cell like the Kynoch foam but way cheaper.
They are also the right size (a little big) and have a flat base that should hold the powder in place well.

Your foam backing rod which I understand is closed cell.
I'll have to pick some up. Looks very easy to use.
Thanks again for pointing me in that direction.
I have a couple of other 444 projects that look to need a filler including regulating a 240 gr JHP in the double rifle using 2400 or IMR4227.