Could it be possible to add the slingloading to the medium container ("Land_Cargo20_brick_red_F" and all the others of the same type...) and maybe adjust is mass (20000) ? (see the image below, the container in the middle).

http://imgur.com/a/UrgmL

It has exatly the same shape that the huron - container ("B_Slingload_01_Cargo_F"), mass (7500).

Actually we can juste slingload the little one ("Land_Cargo10_orange_F" and all the others of the same type...).

Thanks :).