Best material for compression?
Infill has minimal effect on the strength of printed parts, so I would expect the part to break in the same spot regardless of what infill percentage you used.  
PLA is especially poor in this exact application, and it undergoes significant creep/cold flow under mechanical compression over time, so even if achieved the necessary strength by changing settings (which you can), it would require that you periodically tighten the bolts more and more, as the PLA would slowly deform under the mounting pressure.


Perimeter width and number of perimeters are what primarily influence the strength of a printed object, infill has very little impact on strength in comparison, and unless you're using an exotic pattern like gyroid, what impact it does have is not even close to isotropic (will add strength in some directions while doing nothing in others).  But even then, infill only really has an effect when we are talking about forces that are spread evenly over the entire object, not concentrated strength of a specific spot of the part.  And that effect is always much weaker than what perimeter count or width will have.

Just bump up your perimeters to 4 or even more and that should make a huge difference.  

And also, don't use PLA.  I think PETG is a much better choice in this situation.  It is more ductile only slightly less rigid than PLA, making it much more durable overall than PLA, and less prone to cracking under compressive forces.  PLA theoretically has higher tensile strength, but that often doesn't mean much.  

I would not recommend ABS, it tends to have similar issues with brittleness and is one of the weaker materials one can 3D print.  

It terms of ordering a special high strength polymer.... unless printer and hotend is rated for in excess of 400°C, no such 'high strength polymer' exists, at least not that you can print.  PLA and PETG are close to the best you can get, with Polycarbonate inching out ahead but not by a huge amount (~20%).  Despite what filament companies would like you to believe, carbon fiber reduces the strength of PLA, ABS, PETG, PC, and probably nylon, and instead simply makes those polymers more rigid and increases dimensional stability.  The only filaments that would actually be made stronger with added fibers short enough for filament manufacturing processes are ones with glass fiber.  But you don't want to print those filaments, trust me.  They will dull the teeth on your hobbed gear(s), even if made from steel, and will just ruin all but ruby nozzles very very quickly.  And they still wear out ruby nozzles even then.  

There are exotic polymers, but none of them print at less than ~350°C, and are generally exceedingly expensive.  All the polymers that can be used at normal printing temperatures are all fairly similar to each other in terms of tensile strength at least.