Waterproof temperature sensor housing

I had some plates made at work. They are 60mm x 60mm x 10mm copper that is then tinned via electroplating. The electroplating is to stop a layer of copper oxide forming on the surface and acting as an insulator. Between each layer I have inserted a gasket made from a bicycle inner tube. The hole in the middle is 18mm and is the cavity in which the LM35 temperature sensor and an LM358 op-amp with appropriate resistors (270k / 27k) for setting the gain (1 + 270/27 = 11).
I tested it for waterproof and all went well.

Temperature and op amps

As I said, turns out the conductivity sensor has no built in temperature sensor is in, which is a pain as I have to make a waterproof thermometer. No matter, I have the sensors - LM35. My LM358 Op Amps arrives (5x) and with them my 5v regulators L7805CV.

Even though the function for calculating the gain of an op amp is easy Vout / Vin = 1 + Rf / Rg, finding resistors from the box of resistors you have takes a bit of time.

I ended up choosing 2 x 270k for Rf and 55k for Rg. I don't know if 2 x 27 and 5.5 would be better. But I ran it in the Livewire simulator and it worked so we'll see.

Now I have the regulator I am going to include that too.

As usual the circuit is online. http://www.circuits.io/circuits/6588

G = 1 + Rf / Rg = 1 + (270k + 270k) / 55k = 540k / 55k = 10.8182

Using this ratio we can see that we top out at just over 30 ° C. An unlikely value for a tank of water in the UK

A quick test with that circuit and I get 1.5v in my nice cosy house.

° C	LM35: 0.01V per ° C	LM358: * 10.8182
0	0.00	0.00
5	0.05	0.54
10	0.10	1.08
15	0.15	1.62
20	0.20	2.16
25	0.25	2.70
30	0.30	3.25

Now the problem I have is how to make this :

waterproof!