The wedge is a straight taper, nothing cut off the sides. I think you are looking at a stain line, not a cut. Some times to get a tighter fit, I will cut a saw kerf in the base and drive a wedge in it to expand it a bit.
I set the temp. in my Walmart fry pot to 320 degrees with a candy thermometer. The thermostat reading are highly unreliable. If I am doing only a few boxes, I use a heat gun and heat each one as I go. Get then hot enough that you can only touch them for a split second. Tap them on the work bench, the sound gets duller as you get then up to temp.