Matt,
That is quite the piece!
It is possible to put together home-made Mar Bars or the single line version, called Gleasy Bars in my experience. However, more than the fair share of attempts I have seen tend to torque the frame of the ascender. That is probably why the pice of wood in your pic is so fat.
Mar-Bars can be used on a single line. Usually, I place the top and bottom ascenders on opposite sides. That is to say if the top ascender set is hooked into the right ascender, then the bottom will be hooked into the left. I sort of tweaks the set a bit. I don't do it that often as I have better single rope ascent set ups. Mar Bars excel at long open canopy ascents.
When constructing devices, be sure to never modify the ascender. No extra holes, etc. It is no accident that the original Mar-Bars use four CMI ultra ascenders. The frames are heavy.
Good luck,
Tony