> Do you have any tips for holding the material in place when sewing flat felled seams? Pins, paper clips, glue stick, silicone glue?
For silnylon? PINS, very fine pins, and lots of them in the seam are (ie, where they won't affect the waterproofness).
Yes, it does take a bit of time putting all the pins in, but it is the ONLY way to make sure the ends of two curved bits of fabric match at the far end. If you are sewing a complex pattern, this is essential.
Sarah mentioned a roller foot. I find this essential too. You should be able to get one in a sewing machine shop - they are not rare.
As Jay said, sewing with the silnylon fabric under tension is almost a must, along with a low tension in the upper thread.