I. Grab necessary files ("brave.com" directory) from deb package:
$ wget https://github.com/brave/brave-browser/releases/download/v1.60.125/brave-browser_1.60.125_amd64.deb
$ ar x ./brave-browser_1.60.125_amd64.deb 
$ tar -xvf ./data.tar.xz
$ mkdir -p ./tczroot/usr/local/bin
$ mv ./data/opt/brave.com ./tczroot/usr/local/bin/brave.com
$ cd ./tczroot/usr/local/bin
$ ln -s brave.com/brave/brave-browser brave-browser

II. Manually add a few fine touches:
tczroot/usr/local/tce.installed/brave-browser
tczroot/usr/local/share/applications/brave-browser.desktop
tczroot/usr/local/share/pixmaps/brave-browser.png

III. Brave's binaries already come stripped, so just go ahead and create the extension:
$ mksquashfs tczroot/ brave-browser.tcz