This is a model of a simple battery cover. The measurements are on the second image. To mount the spring to the plate, I would recommend to glue it on there using super glue and then melt some kind of metal in the place where you just glued it. Here is a picture to hopefully make it understandable:

The purple things are the prints and the red line is the piece of metal to melt in the plastic. You don’t have to do that, but it just gives the part more strength.