Materials for wax seals

I'd like to get seals made to be used to make impressions in wax. Specifically, the ones here. I was looking at doing it through Shapeways or a similar site, using their fairly generically described "strong and flexible plastic". Is there a reason this wouldn't work well? Is there another material I should use? For some of the designs, it's showing concerns about wall thickness, which is as low as 0.33mm in some parts of the model. Is that something that I need to be considered given how small this will be and what I'll be using it for?

Thanks for any help!

You don't need flexible for a seal.
Traditionally seals are carved from onyx or a similiar very hard material that carves well.

The trick to getting a good impression with a seal is timing.
The wax needs to still be soft, but not too soft or it can stick to the seal.
Many years ago I used to seal my letters with a wax seal :-)

0.33mm is very very thin. Even with sls, that's going to be fragile.