I have held the device on my bike with a rubber band. But now, with the delicate screen attached I need something more protective of the circuit parts.
pages/t-beam-bracket
It will be easier to think creatively about a bracket if we first make a model of the board we are to hold.
I'm sizing components within modules and then placing these indexed to the lower left corner of the board. I'm not trying to cancel out measurement error in definitions. Rather, I use the visual alignment to confirm my measurement accuracy.
Other models. printables 
thingiverse