Film to digital conversion using a camera and Gimp

First try :)
Already tired of my cheap film scanner, that does level adjustment, color balance on it's own, based on some unknown measurements...
I'm trying to shoot the film with a digital camera and then invert it in some graphics processing software (as an open-source fan, I'm using Gimp)
Once I get the right way of doing it - I'll be posting the complete process.
The main problem is in removing the Orange/Brown tint from the negative film, I'm trying to do it by using a custom white balance setting.

Here are the steps I'm taking now:

• put the film between two perfectly clean and transparent surfaces (e.g glass)
• put this 'assembly' on top of a completely matte surface (thin white plastic), which is placed on top of some white light
• find a portion of the film which is completely exposed (transparent), and use it as a sample for setting a custom WB (white balance) on the digital camera (the camera should be placed on a stand btw)
• take a photo of the picture on the film
• open it in Gimp, crop the image (cut away all the unneeded borders), Colors->Invert, Colors->Levels (adjust them to your liking)

Sample picture below:

If you know a better way to do it - I would be glad to know.