Currently using spandex to build a prop, however am having problems with it curling at the edges that are visible when stretched, and I need these edges to overlap to form a flat surface. It's also forming into a slight 'c' shape when being stretched along it's length.

Any tips be appreciated, or some alternative materials that could be used.

Spandex does that.
Try turning it 90 degrees and/ or sewing a second layer under it that's at 90 degrees.
Then, there is a way to stop the. "C" problem, it's in the way you stretch it... Or don't

Try sewing the edges, French seam or enclose the edges with Bias Tape from a fabric store.

Hi thanks for the replies and tips. What I did in the end was fix some elastic strips on the edges, which are stretched and attahced to the frame. It stopped the 'c' shape and stopped it curling in on itself.

That’s good too

That's what I was about to recommend too. Elastic the edges and you're golden!

Does Spandex need to be stitched with an elastic thread and or that clear elastic bias tape?
Metal tubes are going to be pushed through it.