Awaz, instead of relying only on the tape, try using a zip tie or two, to secure the two halves of the case together.
You can solder to batteries with regular soldering irons or guns, but you need to scrape the shiny coating (non-technical term) off, usually down to the copper or tin underneath, and pre-tin the spot you want to solder the wire to. However, as others have stated, over heating the battery is really bad, and could potentially cause it to rupture. If you do this, do it quickly, and use lots of flux.