You have quite a few options to box in a meter and there isn't a best one as such. What you choose to box the meter depends on where the meter is, what is around the meter and how much your budget is.
If the meter is outdoors then treated wood or plastic casing is preferred so it can survive the elements. If the meter is indoors is it in plain site or is it out of sight? If its indoors you have choices of planed pine, mdf (cheap), moulded composite. All of which can be painted if necessary.
Just bear in mind that it will require regular access so a door or viewing panel would be needed so you can get readings.
Hope this helps