Another experiment. Assembled the cube (with the dubious plastic corners), and started thinking about the diagonal braces. As a structural material, plastic is crap compared to metal. Fabricating a bunch of aluminum corner braces is tedious if not hard.
But … the loads taken by the braces are a small fraction of the load taken by the aluminum beam. Could a plastic brace serve? (A proper mechanical engineer could likely use finite analysis to come up with a firm answer. I am only a software guy who once had a degree in Physics.)
So … fabricated some braces out of plastic. (Lost time when my printer refused to cooperate for half the day.) Two iterations shown, and a third printing. Even without tightening the screws (as I need to fabricate a truing jig), this is noticeably stiffer. Right.
These braces are (at present) PLA printed at 20% infill (at 150mm/s). They feel a bit like balsa wood. (I built model airplanes as a kid.)
If my experiments keep working, tempted to call this exercise a “Boggart”, as I keep yelling “Ridiculous!!”, and it continues to refuse to turn into anything evil.
apart from the stiffness, printed parts may also add a bit of damping [this is physics :-)]. So now the next step should be hybrid metal/printed part. Bring out the FEM!
@Preston_Bannister I would like to offer some constructive criticism. These corner braces seem to be designed as it they are to be fabricated from metal - you have a 3D printer! You can design and print the braces with additional features to help their function. For example, why make a separate truing jig? Make the braces with runners that fit tightly into the extrusion slots or over the bar - instant, built-in truing. And while they look quite technical, the lightening holes don’t need to be round (as if drilled), almost any shape would work - polygons (with radius corners), ellipses, or something organic. I love the idea of more printed content in 3D printers and I agree that the damping properties of plastic may be quite helpful. Keep up the good work!
@David_Sherwood hardware like you reference has been proven to make structurally sound machines; but, I think the point here is to see what is possible using printed parts. It’s the thrill of the chase so to speak; and along the way, something new or novel might be discovered (built-in dampening for example).
I too have been known to still just place holes for material relief simplicity. And I also have the same kind of distractions . One concept that I’ve been wanting to play with is TO. Your corner braces would also benefit from this, even if just mimicked, and not computationally generated. https://en.m.wikipedia.org/wiki/Topology_optimization
@Alan_Thomason I agree. Did I mention my CAD skills are quite recent?
Keep in mind I am trying the simplest thing that could possibly work, fabricated as simply as possible. As I expect to iterate, my i3 clone is doing prints at 150mm/s to save time. In truth, the simple holes in the brace were added mostly to save print time.
@Preston_Bannister I figured that either reduced print time or reduced material was the goal with the holes. I frequently iterate designs in the same way. Keep us updated, I’m really interested in the stiffness/damping properties that result.
Plastic is good or better than aluminum if instead of replicating aluminum braces design you could think out of the box and come up with a dedicated plastic braces design.
@Alex_Koukarine Plastic is certainly easier to fabricate into complex forms, and that can be a huge advantage. But as a raw structural material, metal is far superior. Clearly I am trying experiments to see what can work (or not).
I am particularly wary of creep under load. Those curves tend to be roughly exponential in shape, and if load is well-distributed and on the safe part of the curve … plastic might be good enough.
In part, that is why I am starting with very light prints. If they seem sufficient under test, then I might be much more confident with a heavier print.
@Nelis_Willers I am very concerned about the effects of resonance, as I aim to push both speed and size. That dubious plastic corners and diagonals might help is … well, a bit odd.
Note that also I made my desktop PCs near silent more than a decade ago through use of a dense, acoustically dead material. Translation: I stuck vinyl floor tiles (from the hardware store) on the inside surfaces of my 1990s era PC Power and Cooling steel cases.
. One concept that I’ve been wanting to play with is TO. Your corner braces would also benefit from this, even if just mimicked, and not computationally generated. 
