In college I worked at a frame shop, so we used "framers corners" to hold artwork/photographs in place behind a mat before framing.
I adapted my envelope sheets for my light box with guidelines that I design, print off and add the two framers corners at the bottom. I just slide in envelope and I can see the guidelines through the envelope with the back light. Plus, the paper at the bottom helps my hand not stick to the light box surface so I can glide and flourish without problems. I print on using card stock, so it's a cushion for my lettering.
You can find the "framers corners" at art stores, in the DIY framing section.