I doesn't matter what you base it on. Your opponent will still be base to base when his model(s) is touching one of the Defilers legs. I suspect this will cause problems for you as it will generally be closer for people to just touch whatever base you are going to put it on, and to try to argue that they actually need to make it to the legs in order to assault and not the base you have put it on is going to be a tough sell as nothing else in the game works that way, so I wouldn't bother. Also keep in mind that a Defiler is easy to balance on rough terrain with its legs. If you add a base, it won't be.
And there is no Tervigon model, so there is no Tervigon base. He probably meant a Trygon base, but even as large as they are, your Defiler won't completely fit on it. I suspect you'll need piece together a few bases (like 3 Trygon bases), or scratch build a base of some sort, or find a non-GW base. Seems like a lot of trouble for no discernable benefit.
A CD works very well if you fill in the hole. Any old promo disc for something (wow free trials or AOL free trials come to mind) with a piece of cardboard on top of it or hell just some paper taped on top of the hole then primed/based with the defiler works fine.
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