Well, I suppose it depends on which type of columbarium is it: indoor/outdoor, public/private, etc. A public, outdoor one is quite simple, offerings can easily be left and nearby soil accessed, so I’m going to presume the one you speak of is either indoor or private. There are a number of options!
If you can get away with it, you may leave flowers or other token offerings in a designated location – named in that person’s honor – and return to retrieve them after a period of time. Alternately, if you happen to have light fingers you might be able to lift a little something from the location (it needn’t be anything fantastic, a stone, a chipped piece of plaster or granite/marble, etc.) and name it. In town we have a rather dilapidated mausoleum and I have a number of pieces of chipped limestone from its facade. These may then be used (mixed with graveyard dirt, if the ritual calls) in workings or to erect a spirit altar or totem.
If you seek to build a bridge or connect with the spirit, I recommend the latter – constructing either a small altar and/or creating a Spirit Vessel. It can be a grand or as simple, as small or large as you wish or can accommodate. It can be on broad display or hidden away in a closet, but ideally it should have things that belonged to them – in life – as well as things that remind you of them: favorite flowers, their perfume/cologne/scent, even their favorite food may be left as offering (though, I’d recommend not including that in a Spirit Vessel). I find that – in working with the dead – that earth and soil are integral, especially for a Spirit Vessel. It is the medium of life and death, and whether from their grave or not, it’s a must in my book. You can, as stated above, mix the offerings left with graveyard dirt and use this as the foundation for your Vessel. Traditions vary on offerings, from spiced rum to (unfermented) juice, from padè to unleavened bread. If you have a tradition, respect it as you wish, and if not, do what most speaks to you.
Another type of Spirit Vessel is the doll, and while my process (La Muñeca de Tutela) dictates how to create the doll as home to the familiar, it can also be used to house ancestral and the spirits of the departed – with their permission, of course.
I hope you don’t mind, but I’m posting this publicly so it might be of aid to those in a similar situation & in hopes that others might also be willing to lend suggestions!
The Qedavian Apocrypha 