Thanks for the info. on just what coroplast really is, basjos. I found some information about bonding polyethylene by googling and found this writeup about a specialty 3M product, DP 8005:
http://www.stealth316.com/2-dp8005.htm
He talks about the special applicator -- I'd give just squirting it out on a piece of cardboard and stirring it up a try. It looks like it's available at "drillspot.com" here:
http://www.drillspot.com/products/32...astic_Adhesive
With ground shipping a single tube is about $25.00. I may want to live with my ugly lap joint for a while, or even try some really short sheet metal screws run in from the piece behind the visible surface to see it that's good enough for a while.
Edit to add: Sheesh, thanks again, basjos, for the McMaster-Carr reference. I'll see if the outlet near me stocks the stuff.