+1 for the impulse sealer.
Took a piece of a HeatSheets space blanket and used a Uline 8" impulse sealer, model no. H-161. Started at the lowest setting, and at setting #3 got a good solid weld, with the shiny sides facing out, the orange sides facing together - does not work with the shiny sides facing together. The machine looks like a big stapler and was purchased for rebagging trail meals to save weight. Around $100 in 2004. Problem: How would you create a several foot long seam. Maybe they make something that is not closed at one end, as a stapler is, that would do it. Anyway, it is 100% doable.
Just for fun, snipped off a 2" wide 0.18 mylar strip from the edge of some cuben and tried it. Did not seal until setting #8, and that toasted the mylar (PET) around the bond, making it very brittle so that it shredded when stressed. No good.