What type of seam?

In the pursuit of making more of my own gear, I want to make a TQ for two people to use in a roof tent. I need to join the material due to the width and I'm wondering what type of seam to use. A flat felled seam would be overkill I think - should I just join the two halves together with a running stitch then turn the other way round so the unfinished section is inside? I can't see any real pressure being exerted on that seam but I'd appreciate any advice from those more experienced than me.

That depends on the material, if the material can fray, I'd try to enclose it. In which case I would use either the flat felled or a French Seam. With the French seam after you make the actual seam fold to one side and top stitch. Here are some videos on each seam. The french one doesn't cover the top stitch part, but I'm sure you'll get the idea. I think they both use about the same amount of material.... so all things being equal, I'd probably use a flat felled? Hope that helps.

Personal opinion only -- I don't like the bulk of either the flat felled seam or the french seam in this application.

If you have not cut the fabric yet then cut it with a hot knife to seal the edge. Or if the fabric is already cut sear the edge with a flame. Either way will seal the edge so it won't fray. The other option would be to seam the selvage edges on the interior.

In any event if the fabric won't fray a simple single seam stitch line will do the trick.

There is nothing wrong with the flat felled or the french seams but there are other options.