Building permanent magical traps with GURPS Magic

[Grouchy Chris]

Are there any by-the-book rules on constructing permanent magical traps using the standard magic system given in GURPS Magic?

As I read the rules, a mage can cast, for instance, Link and Fire Cloud to burn anyone who enters a certain area. But this has a few shortcomings: the trap can only fire as many times as the mage casts Fire Cloud, three link spells count as one spell "on" for the mage, limiting further casting, and also it's not entirely clear whether the Link would persist after the caster's death.

The rules on enchantment say (Magic p. 18) that some spells can be used to enchant an area, but Link is not one of them.

So are there any rules using the standard magic system for creating magical traps that last indefinitely and can fire an unlimited number of times, without burdening the caster with a "spell on" penalty? I've looked through the books I've got, but perhaps there's a Pyramid article or something I don't know about.

I'm perfectly comfortable adapting the existing rules to cover this case, but first I'd like to know if someone's already done it.

And yes, I know about Evil Runes from DF2, which create magic effects by fiat. That's not what I'm looking for.