Rather than asking to buy resistors, buck modules, time switches and so on, how about a simpler option?
On the web site selling these, they have LEDs pre-wired with a series resistor for 5-12 V DC:
https://www.modeltrainsoftware.com/smd-chip-leds.htmlThis is nice, because two of these could be put in series off a 12 V battery. Not only that, but at 6 V per LED the current draw would be much reduced from the maximum. (If we say the resistor is sized for 20 mA at 12 V, then the current would be approximately 4 mA at 6 V.)
So now, we could consider running 12 pairs of 2 LEDs from a 12 V car battery for a total current draw of only 12 x 4 = 50 mA. Over 240 hours that is 12 Ah. That might even work running the 5-12 V LEDs individually off 6 V lantern batteries (which have a nominal capacity of 10 Ah for the alkaline version).
Therefore, proposed solution: Use the 5-12 V chip LEDs with pre-installed series resistor. Group up to 8 LEDs to run off a 6 V lantern battery. Use 3 or more lantern batteries according to the number of LEDs required.