I just found a product that so far has worked wonders for me. And judging from reviews on Amazon, for others as well.
Captain Tolley's Creeping Crack Cure.
I've used it to cure a nasty leak in my 944's front windshield. The leak was sucking in mass amounts of water due to capillary action. I've known of capillary action since I was a young Porsche admirer but didn't really know how powerful it is. It will draw water uphill through tiny cracks and in my case was completely drenching my car's ECU's and passenger compartment. After some magic from Captain Tolley, the leak/leaks have stopped completely. I've also had the same experience with the 944 taillights which were filling with water before Captain Tolley. Presently I'm trying it on my 944 rear hatch which some of you will know is a major problem on these cars where it becomes detached and leaks into the rear compartment. Normally, this requires separation and resealing with a 3M industrial sealant. Captain Tolly is trying his best but his proper usage is on small cracks whereas the rear hatch gaps get larger than the Captain is meant for. Still, I can see this solution filling the gaps slightly over several applications (I can see the milky solution creep in using capillary action which is amazing to watch. It then dries clear.) It'll most likely need additional window sealant but it is filling the tighter areas. I'll see if filling a little at a time works out or if a thicker sealant is required.
Anyway, not sure of its chemical or heat resistance so it'll probably not work for a hostile environment such as a coolant tank but worth a look. And also worth a look for other leaks you can't cure; trust me, you'll thank me later!
One thing I can't figure out is why no one has made a metal coolant reservoir replacement for our cars. I've been meaning to look into fitting a generic or maybe a tank designed for a different car into a 986. I'm sure all the connections will complicate things but I worry about other things that aren't so obvious and which would require an engineering degree in fluid dynamics to understand. Any engineers out there who would know if complicated coolant flow is at work here or am I over thinking it? Also, the bleeder valve is more trouble in the design of a replacement. Still, so many other cars have aluminum reservoirs made by aftermarket companies in the $300 range. I always thought that was pretty expensive but in the 986's case, it doesn't sound so bad. And I think there are countless Chinese reservoirs for under $100.