So, my untutored mind says one of the F13-F24 might be the right choice (?)
I do not know, but if you plan to make that key your left shift key, using the scancode for the left shift key might be an alternative.
To make this convenient, I'd perhaps like to have single L-space be a dead key, double L-space be a shift lock, and the shift-lock would be cancelled by a later L-space.
You can achieve this by mapping the keysym ISO_Level2_Latch to your key, and put Shift in the modmap of that key. If you have selected the scancode for F13, it will look like this:
key <FK13> {
type= "ONE_LEVEL",
symbols[Group1]= [ ISO_Level2_Latch ]
};
modifier_map Shift { <FK13> };