I have zero knowledge on this subject so any help is appreciated...

I have a long evening gown like dress that is of thin material and I am looking for a model to wear it but this is where I am hitting a bump....

The models look has to be fairly busty and curvy and I have found a few that fit the body type however their mid section could use a little help in getting an hour glass figure.

I know a corset will do the trick but it will show under the dress which is not what can happen as I cannot Photoshop the lines out, all the pictures are needed as shot and there will be a lot of them.

Spanx might help but are there any other options besides wrapping the heck out of their waist with duct tape?

Duct tape Im hoping was a joke. Do not ever do that.

Spanx and other such control-top shapewear are your only near-seamless and painless option. You can also add a padded/pushup bra instead of trying to reduce a waist

Or better yet, keep looking for someone with the appropriate shape to cast.

Don't try to change the shape of a human being to fit a piece of wardrobe.

Get a model who fits the dress, or get a seamstress to adjust the dress to fit the model.

This.^^^

Find another model.