For a lot of plastics welding with a suitable filler rod is useful, but you also need to add extra support, where I find that a good thing is to use uncoated paper clips, as the steel wire can easily be bent to shape, and you can easily melt them into place to reinforce the crack, and fill it in later on again, with the paper clip providing structure. Also for small pieces a staple works well, melt in to the material across the break, and use a few of them.
For the Scopemeter the plastic is probably a glass filled one, and there you often can get by using strips of a similar colour PVC which you melt in together with the original. For the stand off best is to forget about rebuilding them but find a existing insert in a similar size, and remove it from the sacrificial case, and melt into position in place of the original, otherwise I find for the cracked ones a few turns of thin copper wire wound around the outside, and twisted together, and then with a coating of contact adhesive applied, to bond the wire to the plastic, works well to keep the crack from spreading, though you can also use a drop or two of acetone after the wire winding to melt the existing plastic slightly, fusing the wire in. Works on self tapper crew holes, and keeps them cracking further in a lot of cases, but does not work on ABS, where you are best off making a replacement one out of a donor, and either using 2 part epoxy to attach it, after a wipe down with IPA and some sanding, or weld in place with heat.