SSRs can get warm, depending on the load they are switching. You wouldn't want the cover to melt, so PLA is probably not a good material to use for this. The body of the SSR appears to be ABS, but it's hard to say for sure. I used ABS which won't melt until the SSR gets so hot that it fails anyway. PC would be a good choice of material for this, too.
This is one of the last parts I designed using DesignSpark Mechanical as I am switching over to Fusion360.
The original design and STL files are here
|CAD image of the cover. Walls are 2.4mm thick, holes fit #6 or 3mm screws. The slots are large enough to fit over lugs, or you can just screw the wires to the SSR's screw terminals.|
|Photo of the printed cover in the printer.|