Use a fairly substantial piece of gauge plate or CRS. Heat it until it is hot enough to melt the belting after no more than a couple of seconds. Hold the ends of the belt at right angles to it for four or five seconds to get them well melted. Now hold the ends of the belt together firmly, without moving for a minimum of five minutes, preferably ten.
It's most important to keep the pressure on and avoid movement for far longer than appears neccessary. It takes time for the centre of the material to solidify and it takes more time for it to cool enough to have any strength.
Then leave it for a while longer, trim the flash off the joint and it should be ok.