Mitch,
The thread is quite long as I answer this.
Solder through the cap -- Right! You can clean and dry the cap dependably.
U-tube of suitable fuel line closes vent and fill tubes. Silicone is NOT suitable for diesel.
You only need remove the tube to the fill line, a good defense against evaporation, moisture absorption, and dirt entry. The whole cap only comes off to refill the can.
For a seal on the cap threads, plumber's teflon tape does very well. As you're not constantly removing the cap, it stands up as long as you need.
Mark the cap some way to distinguish vent and fill lines. Pulling the vent end first WILL create that fuel geyser. (We usually squeeze the line we're removing, so any pressure remains trapped.)
I tote canned fuel in a cardboard box about the size of a computer paper box. Painted inside and out with (probably) Krylon, or a quick-dry "refrigerator/porcelain" type... it's been a while.
A tote-handle of half inch nylon strip, the stuff like metal baling bands, bolted through a pair of reinforcement disks on the ends has held up for years -- surprisingly.
Paint a light color to reflect heat; cardboard is a decent insulator. Fuel stays cool in Summer, and warm in Winter. Fuel temperature has a surprising effect on setting drift from cool mornings to hot afternoons...