A pack of Pro-Create modeling putty - http://www.kraftmark.biz/kraft.procreate.html
It comes in a practical plastic box with lid.
As you can see it comes in two wrapped bars - one is the resin and one the hardener.
So I gave it a try and took two even amounts of each and mixed them just like GS.
It is softer than GS, so you may have to adjust your sculpting and modeling procedures, but is really easy to work with.
Here I started with the boots for the Ogryn.
The putty smoothens really nicely and is longer formable than GS, which is getting hard - not cured, just hard to form - rather quick.
Next I worked on a kind of powerfist or at least armored fist for the left hand.
it allready has some scars and scratches - I will still have to add rivets and stuff, but actually I'm pretty satisfied with this stage.
Next was the right arm.
I formed the biceps some more, worked on the elbow, as well as the lower arm muscles.
Obviously I also mounted the right hand with the chainsword.
I really like how the Ogryn progesses. The putty remains workable rather long, but seems to cure faster than GS.