Why not use the chafing dish warmer on high? Melt a little butter, add the marshmallows, slowly stir into a smooth creamy mess of melted marshmallow and then add the crispies...since the warmer works like a double boiler or baine-marie the temperature is around boiling or 212* Fahrenheit. I even put a lid on the pan and when I got busy an came back to it later.
I used a standard recipe x2 which yielded a half-sheet pan of crispy treats.
6Tb of butter (3oz)
20oz of small marshmallows (two 10oz bags from grocery store - about 4 cups)
12 Cups of crispy rice cereal (3 quarts)
Melt the butter down, add marshmallows, melt/stir till smooth (you can take your time on this part), fold in crispy rice cereal, use non-stick food spray to cover half-sheet pan and then smash down mixture to fill in the shape of the pan. Probably will also need to spray hands to keep mixture from sticking to them. If needed, spray the bottom of another pan and put on top to smash down evenly.