I was following Sheldon's example: I've made fixed conversion by brazing the cassette freehub body. It's an old Shimano 7 speed. I've took the freehub out, soaked it in degreaser, dried it, and a friend of mine brazed it. I am riding that bike as a commuter/beater since 2013 with no problems at all (unscrewing), even the bearing cup is still OK (I was afraid that during the brazing will loose it's hardness).
Taking apart the freehub mechanism is tricky, requires special tool. simply soaking it in degreaser and then drying is simpler. I think that welding could cause small deformities, brazing will not. That's why I chose brazing over welding.
I recommend you my method, if you want cheap fixed gear conversion. Chainline adjusment is very easy too. I rarely make skid stops, and run a front brake.