Fusion 360 is probably easiest, but you'll have to create those holes individually so far as I know. You might be able to do something programmatically in OpenSCAD if the holes are sized according to a pattern. Personally, I'd go with Fusion 360.
Pretty straightforward to model in Fusion 360. Plenty of great tutorials on YouTube, everything by Lars Christensen is top notch.
No need to model all holes separately, at least as long as they are all the same in one row (that's what it looks like). The Rectangular Pattern command is your friend here.
Material: I don't see a reason why PLA wouldn't work for this application, personally I would probably print it in PETG to make it a bit more robust.
Infill, I hardly ever go above 10% unless I have a large top surface that needs more support from below. I usually print 3 perimeters. There's a good article somewhere on the Prusa Blog that discusses infill versus perimeters, just google for it. My preferred infill pattern is gyroid but recently I've used Adaptive Cubic more and more and have been quite impressed with it (not that I have done any serious multi-directional stress testing though).