After a little research, I figured it was probably a good idea to replace the MDF with some dressed (Premium Pine from responsible sources in NZ) pine. $10 seemed a good price to eliminate any cancer causing Formaldehyde resins and other potentially not-so-nice stuff.
This mill mount is made from 140x19mm pine (a single 1.8m length cut into 3) and another smaller-sized 1.8m length:
Same process as before, with the hole-saw:
and jig-saw to cut the mill-mounting hole:
Lots of wood screws and some cross-pieces make it feel very solid:
More screws, and a few washers, hold the bit of wood that will slots under the lip of the keg so that it does not move or tip when in place:
More cross-pieces of wood and screws around the mill, should eliminate any chance of the timber warping or buckling when the grain is loaded into the hopper:
From the bottom, the completed mill mount looks like this:
And from the top:
All it's missing is some Velcro-straps that will hold the drill in place.