I using DJI Flame Wheel frames as workhorse for my experiments with drones. They are really robust, cheap and lightweight. Lack of room for modules is the only problem with them.
So I implemented 3rd floor board to mount FMC and companion computer over battery and bottom mount for Tarot FY680 landing gears. All STL files are published at Thingiverse
Also bottom mount could be modified to mount a rail system. All parts printed of ABS, they are fit into frame really well and can be used as a base for your custom designs.
Feel free to use and modify it.