Take the Lasgun and cut off the stock from the shoulder to the carriage. Cut off the magazine. Cut off the barrels. Take the Keltagun and cut off the melta-bit and maybe the small promethium tank (if your ambitious). Glue the melta-bit to the end of the gun and the tank wherever it will fit. The
IG hands and arms will fit normally.
This is a trial bash I put together using a bit of plastic tube instead of the melta-bit.