Farm jack base plate

Any of the 4wd shops sell them Allan. Also on ebay if your not near a retail outlet

G'day Allan, we have a genuine Hi Lift it has two holes in its base, what I did was cut a piece of Steel chequer plate (I think it is 6mm thick) 300mm X 300mm drilled two holes in it and then welded two bolts in the holes to locate the jack on, they are flush on the bottom face and protrude about 30mm ish for the jack to locate onto (bolts are approx 12mm diameter the biggest that would fit through the jacks foot). I don't have a picture of it, but if you want one I can put it on here tomorrow if you like. It works fine, cheers. Kanga.

You can also use a maxtrax as a base if you have them

100 x 245 is a pretty big base already. I can't see the advantage in putting anything under it.
I use my highlift jack a lot (it jacks the Tvan, the vehicle and I use it as a beadbreaker) and never found the need for a plastic base.