keyboard - Space as modifier key XKB

Background:

Ultimately, I'm trying to set up a custom one handed keyboard layout similar to this demo(click one-handed typing demo).

XKB is used in many linux distros. Wikipedia has a decent description of xkb.

In human-computer interfaces, the X keyboard extension or XKB is a part of the X Window System that extends the ability to control the keyboard over what is offered by the X Window System core protocol. The main features of this extension are:

Unfortunately to achieve this functionality I must make the space bar a modifier.

The Question:

How would you set up the space key as a modifier in the XKB program?

http://www.charvolant.org/~doug/xkb/html/node3.html

It appears the space is unable to be a modifier by default.