How to attach a plastic jerrycan as a FRONT bicycle rack? (How to build a DIY basket-type attachment?)

Sounds to me like you want a front rack, and then to mount your container on that. There's unlikely to be anything available retail that matches your needs, so I see some engineering and modifications in your future.

Rack Exactly how your rack is mounted depends a lot on the bike's frame and fork, in what mounting points are offered.

1. Fixed fork, no suspension, nutted or QR axle. This basic idea would be my preferred solution.
   _{from http://www.hembrow.eu/frontrack.html}
   This provides support from below via two struts, and upper support/stability using the handlebar. Will not work with suspension.

2. Here's a bike rack for a folding bike - this only secures to the frame, and likely is only good for a kilogram or two so is unsuitable for your requirements. Not suggested.

3. Some front racks secure to the forks with clamps - this is kinda scary because forks can taper so anything clamped around them will still slip down, potentially locking your front wheel. Not recommended - you need bolts and security.
   

4. If your bike has cantilever/V-brake mounts, it may be possible to use them for support points. Here's a small lightweight rack that uses the brake bosses and the center bolt for a triangle of support:
   
   ^{From https://forums.mtbr.com/surly/front-rack-photos-please-770014.html}
   This guy is definitely using disk brakes, but his fork offers all three brake mounting points. Again, this one doesn't look strong enough for the full load you might have to carry.

• Do notice that all of these racks slope back or are just-about level. None of them slope forward at all. This is about getting the Center of Mass over the Axle.

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The box

Now you specifically want to mount a "jerrycan" made of plastic. That would be somewhat useless by itself for carrying anything but liquids, so you probably intend on cutting an access.

1. Hole in the side:
   
2. Open like a clam/suitcase:
   
3. Flip-top lid like a lighter:
   

• Consider how you will make your rack box waterproof and secure. You might create a door with a lock, but by the nature of cutting there will be no overlap for a seal. You might look at rubber edging to go around the outside of both parts.

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Securing the box

1. Bungie/elastic ties around the box, both fore-aft and transverse should hold it down against light weights. But 5 kg inside is a bit much.
2. Motorbike tiedowns - these are a little more secure.
3. Consider permanently fastening the box to the rack - you could use saddle clamps around the rack's structure, and then a pair of bolts through the box, with a good-sized washer on the inside to spread the load. Four would be adequate.
4. If you want the box to be removable, look at how jerrycans are held down on off-road 4WD vehicles, both civilian and military.

If your front rack had sides like this then the jerrycan would simply slide down into it, with a single retaining strap over the top.

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• Avoid reusing an old jerrycan that has held fuel if possible because of contamination and staining, and especially if you carry food in it.
• This is not going to be very aero, and if vertical it will catch side-winds from passing vehicles and from nature.
• This amount of material will change the balance and handling of your bike even when empty, and significantly when its loaded.
• If you build this, expect that it could have to carry 25 kilograms, a jerrycan full of liquid. That's a lot of weight so put the center of the jerrycan directly over the front axle.
• Rebalance your bike by building two of these, one for the front and one for the rear of your bike.
• A heavy front rack will make your bike "tippy" and on a hard stop will want to nosedive, throwing the rider forward.
• Consider uprating your brakes, or at least fitting good brake pads like Kool Stop.