Your "seabuilt plate" looks to be an access plate. They are placed on tanks so you can access the interior of the tank for cleaning.
I believe if you unscrew the bolts and pick it up off the tank you will find it is just a plate. Here is the website and this is how they define the plate..
A New Solution to an Old Problem...
The SeaBuilt Access Plate System is a one of a kind product that allows access to your diesel fuel, water and holding tanks - so you can perform the important task of cleaning - giving you peace of mind. Clean tanks mean better and safer boating.
Now to address your problem of fuel gauge.
I agree with
@Stu Jackson, not enough can be said to encourage you to learn your engine fuel consumption by record keeping of hours you run the engine and the volume you add to the tank/tanks. It is the surest way of having the fuel you need when you need it.
That said the convenience of a gauge (even if it is notoriously wrong) is a nice feature.
There are several designs that engage a float of some type, a power source and an electronic user interface telling you the volume of fuel in the tank. To do this you need a Sensor, power to energize the sensor and a gauge.
Starting with the gauge... Choose your favorite marketplace..
Wiring to connect power to the sensor and then back to the gauge. Often times the wire is connected to the back of the key so that it comes on when you turn on the engine. You can also just wire it to a switch that you turn on or off as you need.
Finally a sensor. Here are two basic types of floats in use.
or
Here is a basic wiring diagram for installing the unit. You could drill a hole in the center of your plate and install the sensor through the plate. Since you can remove the plate from the tanks that means you can do the drilling away from the tank This would prevent any metal shavings from falling into the tank. An important step.
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