Made more heaters with plates of different thikness, to test temperature difference. Thermal camera shows only general picture, so used K-thermocouple via drop of rose metal to check most hot (~ center) and most cold (corner at wires side) points. Checks have been done at medium (~ soaking) and high (~ peak) temperatures.

4mm:

• medium: 176-181°C, Δ = 6°
• high: 263-271°C, Δ = 8°

3mm:

• medium: 184-191°C, Δ = 7°
• high: 272-282°C, Δ = 10°

2mm:

• medium: 163-170°C, Δ = 7°
• high: 269-282°C, Δ = 13°

As conclusion - 4mm plate is good for any cases. It still can be cooled fast enough with fan.

Pictures of winner:

What's next

PCB is already complete: https://easyeda.com/reflow/reflow-micro-table. But i'd like to draw device case and verify PCB outline and other things.