I use the program "xcape" [1] to map CapsLock to Esc when no other button is pressed and Ctrl when it is pressed together with another button.
Similarly you could remap the Spacebar to e.g. R_Shift with Xmodmap and let xcape emit a Space when R_Shift (which is now the Space bar) is pressed in isolation.
Similarly you could remap the Spacebar to e.g. R_Shift with Xmodmap and let xcape emit a Space when R_Shift (which is now the Space bar) is pressed in isolation.
[1]: https://github.com/alols/xcape