Dapol actually more ready to run wagon underframes, namely 2x9', 10', 11' and 2x12'. In the case of the latter, one of these is for the grampus but they don't actually advertise the fact. However, as a grampus body will not fit on a normal 12' underframe, which I found to my cost, they will sell you the right one, you just have to be persistent.
Stephen