Jim,

I got ahold of some fairly thin wall steel tubing with just about the
same OD as the magazine tube. Welded plugs in one end. Other end, I
chamfered a bit on a belt sander, to help center up in the magazine
tube. Cut a slot about 1/4" back from this end, about 1/2 way through
the tube. Bent a narrow piece of sheetmetal about an inch or so long,
with about a 3/16" lip at one end. This fits into the slot. Tack welded
the other end, allowing it to be pulled back far enough to allow
cartridges to pass through.
Of course, the speedloaders take just as long to load as the magazine
itself. But in the field, it does allow for a pretty rapid reload. I had
some jeans at the time with a deep narrow pocked on the side of one leg.
Just about right to hold half a dozen loaders.

Jon