The Breadboard Spring Vise makes it easy to hold circuit boards of various sizes in place next to your breadboard. It has no trouble accommodating larger, full-size Arduino Uno or Leonardo boards, and it has no trouble grabbing onto a diminutive ESP-01 ESP8266 board.
The Raspberry Pi fits in the jaws, and it stays in place quite well, but the micro USB and HDMI ports prevent the outside of the board from landing correctly in the grooves of the jaw. The "version 3" jaws are narrow enough at the top to allow a USB cable to be plugged into the Pi.
You should also check out the Simple Component Box for Breadboard Spring Vise!
I am using #32 rubber bands.