That's a lot of questions! Still, let's see…
- If I use this container correctly, am I still incurring significant risks that are hard to control?
Define "significant" and "hard to control". If you are a patient, thoughtful person, it's my opinion that fuel containers can be filled and handled with reasonable safety. It's not as ideal as if you were filling a vehicle's fuel tank directly, but the risks can be managed.
The fact that you even bothered to ask the question suggests that you're capable of, if not naturally inclined toward, the behavior that will minimize risks.
- Is there a certain kind I should get? I am looking at ones made of HDPE with capacities of 15-40 liters and attached spouts.
Any approved (e.g. by your jurisdiction's department of transportation) container should be fine. Keep in mind that as the container size exceeds 20 liters (about 5 gallons), weight will start to be a concern. 20 liters will be about 16 kg (about 35 lbs), which should be very comfortable for most people to carry or lift into or out of a vehicle.
Beyond that, your mileage may vary. 40 liters would be a significant strain for many people, and for some it's more than they can lift at all. Of course, for some people, it'd be no big deal at all. Know your limits. :)
- If I use this container correctly, are there particular steps I should take to ensure safety? In particular,
- Is it safe to store the can in my trunk? For how long?
- My plan is simply to fill the can from the diesel pump at my gas station, store it, and then fill my car's tank straight from the can. Am I missing something in this plan? Are there safety measures I should take when doing this?
- What should I do if it spills at a gas station? What should I do if it spills in my car?
Don't store fuel in your vehicle (except in the vehicle's own fuel tank, of course). Use your trunk for transport only. Some utility vehicles have space for storage of fuel cans on the outside of the vehicle. That would be more acceptable, but IMHO still you would not want to leave full auxiliary tanks even on the outside of a vehicle for extended periods of time.
In terms of filling the container at the gas station, the biggest thing to be aware of is the static electricity that is generated by the movement of the fuel through the pump hose and nozzle. This static electricity can build up on plastic fuel containers, and then generate a spark between something grounded (like the pump nozzle) and the container.
For this reason, make sure that you always have placed the container on the ground before you start filling. Plastic is an insulator, but surface charge can dissipate away to the ground, minimizing the risk. You can also reduce this risk by keeping the pump nozzle in contact with the container, as well as by pumping at reduced flow.
Diesel fuel is a lot safer in this respect than regular gasoline, having much higher vapor pressure and resistance to ignition. But it's easy to take proper precautions, so you might as well. If you are pumping gasoline instead of diesel fuel, these precautions are essential.
Never leave the pump unattended. Your hand should be on the pump nozzle grip at all times while the fuel is flowing. Keep an eye on the fuel level in the container, and reduce the flow to a minimal rate as the fuel level approaches the marked maximum level on the container. Don't exceed the marked maximum level on the container. I.e. don't "top off" the container. It's marked that way for a reason.
If you follow these procedures, the risk of a fuel spill should be minimal. It's unlikely to happen. That said, if you do spill fuel, the procedure is the same as for any other fuel spill at the gas station. That is, if enough fuel has spilled to present a hazard, let an attendant at the gas station know. They should have a "spill kit" on hand, or at least the proper material to clean up the spill.
Again, spilled diesel will be much less hazardous than spilled gasoline (with little to no fire hazard), but it should still be cleaned up. A small spill, say a tablespoon or two, is no big deal. But a few cups or more is an environmental hazard, as well as being potentially harmful to the pavement.
Assuming you close the fuel container properly and handle it correctly, there should be no real risk of a spill in your own vehicle. If you're worried about that, consider getting a secondary plastic bin in which you can set the fuel container, so that any drips or whatever won't actually get on the interior of your car. If you do get fuel on the interior of your car, you can use steam-cleaning to get it out of carpet and upholstery. Better to just avoid the problem in the first place though. A plastic bin is probably a good idea regardless, since even a few drops of fuel on the outside of a container, if transferred to the car interior, can leave your car smelling like fuel for a long time.