My PVC is 80mm in diameter, whch works out to just over 3". I use the end caps that you can buy to fit the pipe and just cut out a piece of laminating plastic to fit into the cap to line it. Be aware tho, some end caps fit tighter than others. The ones I currently have are a little loose, I think because the PVC thickness of my pipe is less than others (does that make sense?)
Anyway, the first time I used my current PVC set-up, I just put the end cap on, poured my soap, and then thought 'Oh, look at that, it's leaking'. I tried to move it to a spot where it didn't matter if it leaked a little, but only lifted the pipe, not the cap on the bottom. I promply discovered just how much mess you can make when the end comes off the mold and you have raw soap batter EVERYWHERE
. Needless to say, I now tape my ends on with packing tape. Just a tip.
I also use a sheet of laminating plastic to line the actual pipe. I tried using baking paper ONCE - it stuck like glue!! I had to leave it for about a week before it would come out.
I find that by using the plastic, it slides right out, then you just unwrap it from the plastic and hey presto! Also, the plastic is re-usable, just give it a wash in hot soapy water, rinse & let it dry.