An example maxtend configuration for an ErgoTravel keyboard can be found at https://github.com/mhantsch/ergotravel-zmk-module.
The detailed keymap is configured in https://github.com/mhantsch/ergotravel-zmk-module/blob/main/boards/shields/ergotravel/ergotravel.keymap.
The module is ZMK Studio enabled, so the configuration can be further adapted using ZMK Studio.
Note: ZMK Studio is still under construction - Status and list of available features.