Diatone came up with an interesting ESC – Mamba T40. Diatone Mamba is a super slim ESC with the only 10mm of the width. It fits on the thinner arms of the quads. This ESC has BLHeli_S firmware and is capable of 40A constant and 50A burst current outputs. ESC comes with metalic shield that acts as heatsink and guard from prop strike. Pretty cheap (from $10.99) if you count the metallic shield into the price.
Available @ Banggood: https://www.banggood.com/Diatone-Mamba-T40-40A-…-1287204.html
@ Surveilzone: http://www.surveilzone.com/DIATONE-Mamba-T40-ESC-3-5S-10mm-Width-g-2187
Input voltage: 3S-4S
Continue current: 40A
Burst current: 50A (10S)
Size: 10×33×5mm (Only 10mm width)
Internal resistance: 0.85mΩ High Efficiency