The pressure gauge can be made up from plastic tubing and 'home brewing' suppliers typically have 'Tee' joints and taps.
My gauge uses 6mm inside diameter tubing and a tap and 'Y' connector fitting from Boots home brew, sadly Boots no longer make these.
Coloured water is added until it reaches the '0' level on both sides. When the tank is pressureized, the water levels need to go to -50mm and +50mm which gives 100mm differential. The drop over 20 seconds is measured, it needs to drop to no less than 50mm differential, which means no less than the -25mm and +25mm marks.
In addition to the basic pressue guage, you may need hatch lids with bung hole fittings glued in (for testing tanks without bung holes) to fit the different hatches you come across. Also photographed is a bung adapter (for boats with non-standard screw in bungs). This uses 6mm outside diameter plastic tube that is a snug (i.e. airtight) fit inside the pressure guage tube. I use a small syringe to put the coloured water into the guage. I also have a bung fitting with the end sealed (tied onto the top of the gauge) so I can check that the gauge is working correctly.