/jp/ - Otaku Culture

I haven't owned a doll in ages, but the lilith uterus in normal was what worked out best.

Since it's PVC and it's inflated, it doesn't have much "give"; this being that, soft hole lose a bit of their interest.
Also avoid any hole based on vacuum: it will not work out anywhere as good as you think, in fact it'll be trouble.

You could give the kashin 六 a try( http://en-nls.com/pict1-14651 )
I can't guarantee best results though.

But what you should look for is: little/no vacuum, medium/strong stimulation, normal firmness (not soft nor hard).
And of course it shouldn't be too long or wide so it can fit inside.

Here's a tip that will save you time and deception: prepare something to fix your doll before it breaks. Do a good research job beforehand, some ways work out much better than others. There are products designed to repair them but I'm not sure what they're worth, I personally used PVC glue and used cut bits of one doll to fix another, and it wasn't really great as the drying time of the PVC glue was 12 hours.

The parts that are a pain to fix are the jointure in pvc.

Alternatively you can fill up your doll with cotton/polyurethane moss(the soft spongy kind, not the hard one for insulation; well unless you prefer that)/ old clothes (cheapest? method but really meh).
It's an investment but it seems like it's worth it(?)
There are a lot of blogs and wikis dedicated to that (in japanese).

Alternatively, you can do pic related.