As Hanaldo mentioned, fluted signboard is quite flexible in a single direction and can be useful where you need to create a bit of an angle between the section that's stuck to the back of the part and the flange itself

I prefer to use 1mm polypropylene sheet, where possible, as it's self-releasing and can be easily cut with scissors into the widths required. With a little heat applied, it can also be formed around corners

If you have narrow return sections all the way around the part, you can bond some foam blocks to the back of the part and sand them level with the edge of the parts' returns. Whatever material you then choose to make your flanges from can be bonded to the foam blocks to create flanges that are perpendicular to the parts' main section.

A hot melt glue gun is very useful for this task and everything can be quite easily pulled away afterwards. The more blocks that you apply will help make the bonded flange material more stable and likely to stay put when you apply filleting wax to the transition line between the part and flange.