If you actually run the calculations, the IRIS-T SLS has a fuel density of 1.185 kg/L, and if you then multiply that density with the approximate volume of the IRIS-T SL expanded motor section (9cm by 239cm) , you arrive at at about 72kg of fuel, which is higher than the 63kg Gaijin has given it, but within the margin of error of napkin math
Relax the entire luncher gross mass (with the missile in it) is about 380kg and it can be almost seen in here (red box) so there is no way for the SLM to weigh 240kg.
Bc if you divided 140kg by 4 this will mean each face of the 4 sides of the container will only weighs around 35kg or maybe less since the container will contain other components that contribute to the complete weight, each container is almost 3.8m to 4m in high now if you distributed the 35kg on length of 3.8m to 4m you will end up with a very thin piece of metal that will most likely not be able to withstand the heat of the boosters. That’s my logic behind it.
So 155kg for the missile and 225kg for the container sounds more reasonable.
We have no concrete evidence for weight, the best source is this polish defense article that has 2 books as sources that describes the development of the SL/M, with it stating that the SLM is projected to weigh 140-150kg with projected max range of over 35km and over Mach 3 speed.