I’ve made several different types of sigil offerings to the spirits. MY preferred method is to burn and be done with it. Sometimes, Leviathan wants something different that doesn’t involve burning.
Once, he wanted NO sigil, but a coarse sea salt and water offering that was to evaporate outdoors.
Another time, he wanted me to place the bloodied sigil in sea salt water and let that evaporate. Afterwards, he wanted it flushed down a drain to be returned to the sea.
Typically, he’s fine with a burnt offering, but there are times when, for whatever reason, he wanted something different. Point is, burning isn’t the only way, regardless of whether it’s indoors or outdoors.