This grow light looks cool, 80x80 seems to be perfect space for one such unit.

These lights have different characteristics and can keep the intensity of light much farther than with T5's, so you can grow taller specimens.

My growbox is limited both in horizontal and vertical direction. I can't have larger cacti, because of such limits. But that doesn't seem to be problem, as I simply cut the scion, leaving few areoles and there is again plenty of space for next round of growth.

Nonetheles the idea about having lophs in the parts of lower light intensity is good, I have one side where is less light and the trichos are growing considerably slower. And though I've never grafted loph, I am considering that.