Many folks have given similar feedback, Unfortunately, the devs have indicated that they will not be adding such storage functionality. Their reasoning (See Timon's response on previous Inno TV videos) is that puzzling out space requirements during the course of the game is part of the fun.
In a way I see both sides. Another partial solution that has been suggested elsewhere is a gallery of sorts. To be able to see (but not place) previous "wins" of event specific buildings. That way, they would not be completely lost. Owner (and potentially other players) could reminisce about by-gone structures and the events in which they were obtained.
The only suggestion I have is really hold these awesome building in the spells area until you are totally sure that you have enough space for them. It is an unfortunate fact that as you progress through the game, such structures that are older basically become superfluous because newer ones are more productive at your higher level. Unlike regular buildings that can be built, sold, and built again, the special event buildings are lost once sold. 'Tis a bit of a shame.
Another comment heard in passing from Timon's videos (s), is that the most popular event buildings do come back (Ex: unicorns, flower cages, glossy garden, etc.). If there is a particular one that is wanted, make sure to reach out to them (via forums??) to indicate such a want. That way, the newer version can be obtained at your higher level and will be more productive than the older version.