Double loading bottom in CNG up / Mii / Citigo ?

The CNG up / Mii / Citigo does not have a double loading base ex works. However, it seems to me that it is possible to equip it after. The bolt holes for the mountings are already there on the sides. However, the Eco floor plate has to be made a bit tighter on each side so that it fits between the mountings. Experiences ?