If you are like me and like having a dozen meltaguns in your army then it's almost mandatory that you get these from the GW webstore. All the other options are just too expensive to consider except if you are particularly good as scratch building.
Upon looking at them you will instantly realise that these are made for Space Marines. The hand that is holding them is armoured and way too big for a guardsman. Also the weapon is quite large and can't fit easily on the model. Some adjustment is required to make everything work.
We should now have something looking like this:
And now for the big moment! Glue the hand with the torso and while it hasn't set yet find a way to fit the meltagun in and glue it to the hand using super glue. Can be tricky but it's the only way to be sure everything will fit together. Use the left hand for the grenade launcher. You might want to cut of the end bit that is sticking out of the fist or else it will push the whole arm away. Now you have an eager meltagunner to field in your glorious battles.