For really complex burns I have a frame about 16x20 openning with a mesh of fine wires crossing the openning so support burn masks.
I take a smaller sized working print and cut it along the line /area that needs the burn, tape it to the wires, mask the areas of the frame it does not cover, and use this as my burner.
Held higher up in the light path it can give almost a perfect fit for complex burns.
The other thing that really helps with burns is a timer that can be actuated by a foot switch.
Soemtimes I make a rubylith overlay so that I can confirm the coverage of the mask in the light path, slide it aside to expose.
With this I turn the seconds chirper on on the timer and count the burn time in my head during a longer overall exposure to allow the mask to be lined up.