You can't just solder a cover on or you'll get bad microphonics. You'll need to get some wax in there as well so that it at least fills the space between the top of the pickup and the underside of the cover. Now you can dip the whole of the pickup in wax once the cover's on, or you can melt as much wax as you can in the upside-down cover (a hair-dryer on a hot setting should do this) and press the pickup into the cover and continue heating for a while to allow the wax to re-melt (some of it will have solidified when the cold pickup hit it), then let it cool before soldering the cover on (after scraping off any wax that may have got in the joints. You only need a small blob on either side of the pickup to ensure electrical ground conductivity with the cover and to help hold it on.

If you screw the adjustable pole pieces right down to start with, then sticking some tape on the outside of the cover first stops the wax running out without the tape being pushed off by the screws.

I'd use a fine wax like beeswax (I got some from eBay from a candle-making supplier) rather than melt a normal candle. Never apply more than just enough heat to melt the wax to avoid damaging the rest of the pickup. It's tricky to know how much is enough and if you have a powerful hairdryer, then you may not need the hottest setting.