If your spru is setting in 5sec, things are too cold imo. I look at the spru, it tells you when its solid. Pot temps will always rise as you cast, unless you have a pid, the amount of alloy is always reducing. IF casting outside, its going to affect pot & mold temp. Maybe a wind screen. Yes a hot plate is the way to go for heating the mold & any ingots added to the pot. Just be sure it isnt too hot for ingots, huge mess.