Easy/cheap way to make my 2014 Ford focus se sedan hold 20-30 gallons of gas instead of just 10?

No Jerry/gas cans I want to be able to get in and drive and not worry about it, any input greatly appreciated

There isn't one. You'd have to build and attach additional gas tanks to the car's frame, and also connect them to the feed line to the engine, and manage the flows such that only one tank is feeding the feed line at a time.

Plus, you'd need to majorly beef up the rear shocks and struts of the car to take the added weight when the added tanks are full, and even then, your car would not handle as it does now, because you will have seriously changed the center of gravity on it.

So, the answer is simple: No. And, I do have and use four jerrycans. Though I don't use them for longer range during a trip, I use them to carry home gas from cheaper areas.

Easy and cheap? No

I don't know the "2014 Ford focus se sedan" but I would be surprised if I couldn't make it work.
But it would not be legal, and will not be cheap, since it would require cutting and welding both the tank and the floor of the rear part of the car.

The ONLY easy and cheap way is to carry around two 20 litre jerrycans of fuel.

But you'll still have to stop when your fuel gauge reads low to refill from a jerrycan.

A full 20 litre jerrycan is also fairly heavy.

Other than that, you'll need to sacrifice load carrying capacity in the trunk/boot/spare wheel well to add additional fuel tanks. If they can't gravity-feed the main tank then they will also add the complication of fuel-transfer pumps.

Your insurer would also need to be informed if you add additional fuel tanks. They might insist on you paying for an inspection by a qualified automotive engineer before deciding whether to cover you.

Everybody is right to say No, but for the WRONG Reasons. The real problem you have is the RETURNLESS fuel system in that car. If not, you COULD feed fuel into the Return Line to fill your tank when it got low, using a low pressure pump switched from inside the car. You shouldn't want to carry more than a 5 gallon can, that should be 450 miles in that car. And you should want to stretch your legs after that long a drive.

Not all that difficult to put a larger tank/fuel cell in the trunk… Race car parts would do it. Whether or not it would be safe, or legal wherever you live is doubtful.

A fuel cell could probably be mounted in the trunk, but it's not going be a cheap or easy endeavor. It'll cost a couple grand, and you won't have use of your trunk anymore.

Not really anything. After all, you got to stop to eat, pss, sleep, check to see if there still is oil in the crankcase after a day of driving. Yes all cars burn oil and you don't want to get too low.