You can butt glue it together if the mating surfaces align pefectly. You can use dope thinner, lacquer thinner or MEK as the "glue". When applied, any of them will soften both mating surfaces, then "melt" the two pieces together as it dries.

Hold the two surfaces together carefully, then "wick" the thinner into the joint with a tiny brush. Work carefully a little at a time ensuring the halves stay in alignment as you go, and don't let the thinner wick under your fingers as they will leave fingerprints on the plastic surface.

You COULD back up the joint with a 1/4" doubler strip (preferably of the same material) on the inside of the cowl using the same method, but you don't really have to as once the cowl halves are "melted" together, they're not likely to come apart anyway unless they are broken.

SIG sells sheets of the material - check their website by going to www.sigmfg.com and click on "Sig catalog", then "building materials", then "plastic", and finally "Sig Aeroplastic".