Thank you everyone for sharing your helpful experiences.
I ended up trying a few things:
Silica sand - good distribution of moisture, but hard to tell when the sand was too wet
Nothing, moist bag - it was too easy for me to add too much water. Worried I may have drowned some. I agree, need to go light on the misting for this one if I try this again. The simplicity appeals to me.
Perlite - Distribution of moisture ok
Peat moss - Distribution of moisture good, but bag looked muddy
And now I'm giving playground sand a try. I am liking how the sand stays clumped together and moisture levels look more obvious.
Instead of sandwhich bags, I am contemplating using smaller thicker baggies or petri dishes in the future.
Being the first time I've done this, even if I manage to get one seedling, I'll be happy.
USDA Zone 3b; north of city of Calgary; Alberta, Canada
Prairies, Chinook winds, 3500 feet altitude