Hi, Matg. I would stay away from a transparent plastic water tank unless there have been HUGE advances in heat resistant plastics lately. Most plastic radiator end tanks used by Ford, GM etc have eventually failed in use. And they were all black. But several owners have added a sight gauge to the outside of their modified tank. All that's needed is a pair of bosses welded/brazed onto the tank for compression fittings. The only problem is durability: the transparent plastic sight tube will turn yellow, then brown with rust and age in 50:50 antifreeze, so after awhile the water level cannot be seen. Then the tube must be changed. Glass is not a good option due to vibration.
20 psi (or even 26 psi!) is a NASCAR radiator cap and usually causes some other component to fail. Sometimes it's the radiator- the small rectangular transfer tubes in the radiator swell up with pressure and block air flow. Check this visually on your radiator. NASCAR uses special expensive TIG-welded radiators that are rated for that pressure. Street radiators are tested & rated for no more than 15 psi. I have an original Euro rad cap from a GT-5 that is marked '0.9 Bar' pressure (13.1 psi).
I have an old Pantera friend- Mats Gorski- in Sweden and I thought you were him until I saw 'Germany' in one of your posts. Good luck- J DeRyke