Andrew: in my experience, if you have a weed-free soil base, you won’t need a separate base.
The heat of the piles will kill any perannial weeds. The problem might arise in your mature pile (left to mature over a 3 or 6 month period) as invasive weeds could become a nightmare! I have couch (twitch grass) which means that I need a layer under the maturing pile.
I have seen a Youtube video on a 1.2m square pallet bin (4 feet square) arrangement with the metal base of an IBC container (1000 litre tank) as the base of the bin. Perfect for gathering the excess moisture of a bin. I would like to replicate, but I can’t find any damaged IBCs. Collecting excess moisture from a compost bin (by having a metal base) would be useful in a dry climate, where each drop of rainwater and moisture is precious.