Did you get a fan yet? You really need a fan to help even out the temps.FINALLY got my incubator up and running.It stays at 99-100 then drops to 96-97.Thinking of putting some water bottles in to help keep temperature up when the light cuts off.Would this work?
Did you also drill the holes in the thermostat? It helps to reduce the temp swings.
Do you have 1 light wired to stay on constantly and the other operated by the T-stat? This is the easiest solution to temp swings and in my opinion is a must.
Is the T-stat mounted within an inch or 2 from the light? It works off of radiant heat not air temp, try moving it closer or farther away till it maintains a constant temp.
Heat sinks are good to reduce heat lost when it's opened and any solid object will work, paint a bowling ball to look like a fried egg and you'll be fine.

Post a pic and we'll go from there.
Happy Hatching but patience till then.