If a sufficient amount of Magnesium could produce the light, then Stoke the Forge or Inextinguishable Flame should work to keep the costs down, and the Rune of the Forge could suffice to stop whatever we'd be keeping it in from just straight up melting.
Basically, we could ignite a year's supply of the stuff on day one, hit it with a spell to keep it going the entire year at max power, and then just ferry it to the part of the mountain we want the shadow to be on the other side of and open up the container or whatever.
Maybe if we enchanted an object to cast the spell on a flame that it contains, we could extend that out for multiple years; set the entire year's supply of Magnesium on fire, then set it to Maximum Power for a year. When the spell wears off the enchantment auto-casts itself after a few seconds, hitting the flame up for another year at a few seconds less than maximum power. Repeat until the level gets low enough that keeping the fire going isn't good enough anymore, then toss the extra fuel in there and turn the cycle on again.
The only issue is having the enchanted object absorb enough Aqshy, but I'm sure we could think of something; it happens to have a very convenient physical manifestation, after all.