"N" scale Big Boy with DCC....

I’ve started the installation of a decoder on my Rivarossi “N” scale Big Boy and am now stopped by a strange(and funny) problem.Converting the loco to DCC operation is fairly easy…isolating the motor is no problem at all and the frame only needs milling a small channel on top for the wires to go by.That’s all there is to it…a piece of cake.

There’s no sufficient space,in fact no space at all,for installing the decoder inside the loco so I was planning to install it in the tender.I’ve done this in another loco and it works fine.My problem is that I can’t get the tender opened.No visible seam…just like if it was molded in one piece.Judging by the weight,it could be just that or there is an added weight inside.Has someone ever opened one if it can be opened,or will I have to mill out some plastic from underneath?I don’t want to mess the outside.Thanks.