I needed some small I-Beams for a project so I decided to make one of my own. The beam is 150 mm long x 10mm high x 5 mm wide. The holes are 3.0 mm diameter. I am fairly new to CAD, so I this is only my 2nd design using sketchup.

I printed it in PLA, using a raft to minimize warping. I had tried one design that had the flanges perpendicular to the build platform and the web horizontal (like an H) but that didn't allow me to add holes. I had to add fillets to the intersection of flange and web to allow me to print without supports. It worked fairly well.