Here's what i did with my key . ...see pics .
Bought a new key case from ebay like this
https://www.ebay.co.uk/itm/Fits-Volv...wAAOSweDdayPxx probably available cheaper elsewhere ?
Carefully cut open up the old key to get to the immobiliser chip,.. pics should help to locate it !
You can either have the new blade cut by a key cutting/specialist place ,.. or cut grind & shape the old blade to fit in to the new case.
I used my old blade. As well as cutting it to the right shape, it also had to be made slightly thinner to slot in the case, i then glued the blade in to the new case using araldite & also made a small pin from a bit of nail, or screw to insert in to the hole for extra security, so i had to make sure the holes lined up ok . I also used a very tiny amount of araldite on the chip to stop it rattling around in the case.
I did both of my keys about 3 years ago, so they are quite robust .