Buy a lanyard with a cord on it the type used for dive lights, by trident or cetsea sp. Look at the computer and figure out where to run it, often through a hole above the buckle when it is closed. I have all of my computers set up this way, make sure it will hold the computer should the strap break, so dont just run it around the strap.
Put the rubber loop of the lanyard tight around your wrist then put the computer on. I found a vyper someone lost at a popular dive site here, no one claimed it when I advertised it locally. I have a UW-Photo friend who had lost a computer a year for several years until she put on a lanyard. The problem in the Northwest is putting a wrist computer around your wrist with a dry suit and thick gloves on. At depth the shrinkage is extreme and loosens the computer. Many womens and small mens wrists (with suit and gloves) are about as thick as their hands and computers easily slip off.
I like a wrist mounted computer as it is easier to read and I dont look at my air as often as my depth.