I designed a mini e-stop button which fits perfectly on the profiles of my ender 3. 40x40mm There are holes for mounting screws M5 with 20mm distance. For the electrical switching you should use a breadboard button 12x12 like this one:

https://www.amazon.de/dp/B00XW2KD82/ref=cm_sw_em_r_mt_dp_TEEMFQSFFC2VVZSXGV9Z

the bottom and the top prints with no support. For the mushroom you will need support.

Octoprint is running with the plugin "physical button" and sends "M112" to the printer.