Sealing fuel tank filler

I am testing the fuel tanks using the manometer method. I've had about a 3"/10 hr leak. I used soapy water to find a small leak around where I duct taped the fuel filler cap. I'd really like to seal it up so I can make sure I don't have a really small leak somewhere else. I haven't seen anything with the soapy water but I'd sleep so much better at night with a solid water level.

Long story short anyone have a good way to make the fuel filler cap air tight so I can be sure the tank doesn't have a small leak somewhere else.

Thanks

Cover the hole with plastic wrap and insert the cap. the plastic wrap makes a good seal.
Another method is to use plastic shipping tape instead of duct tape.

A temperature drop of the air inside the tank or an increase in atmospheric pressure will also look like a leak when using a 'sensitive' manometer.
Instead, try using an inflated balloon, if it deflates - you have a leak. The pressure will also aid with the soapy water treatment.

Test

Log your temps. You are closer than you might think.
Temp change is .810" total per degree.

I have the fuel tank inside so the temperature has been remaining stable. I tried the plastic wrap idea before coming on here and it also leaked through that. I will give packing tape a try.

Thanks

Filler

You could blow a balloon up so it fits but tie something to it so it can't pop and fall inside.

A stable manometer pressure reading is not a good indicator of whether you have leaks or not (which is why it is not the procedure recommended by Van's).
A perfectly sealed leak free tank connected to a manometer is basically a barometer. It's reading will follow barometric pressure. Add just a few degrees change in environment temp and the reading can change even more.

Trying to get a perfect seal around the fuel cap is really a wast of time. All you need to confirm a well sealed tank as a slight amount of internal pressure and no evidence of soap bubbles at any rivets or rivets joints.
That is a lot more precise than trying to get a stable manometer reading.

Example.... what if during the period of time you were monitoring you gauge, the pressure was reducing because of a very slow leak and at the same time the barometric pressure was reducing? Your gauge would stay on the exact same reading (if the rates of change were the same).

Fuel tank leak

I tried the soapy water and still could not identify leak. Used manometer but kept leaking down.
Finally took tank and put in hot tub . Goofy idea but found two leaks almost instantly . One in one of the fittings and one in an end baffle which I fixed through the fuel sender unit hole. But hot tub found the leaks in seconds!
I cold not think of a more effective way. Good luck