There is no requirement for an enclosure when printing ABS. Like many things in FDM, there are improvements to be made, but there is a scale of what is possible.
A heated bed is much more necessary (for similar reasons, the thermal expansion is significant and without a heated bed you have very high risk of warping).
An enclosure is important for high quality, large ABS prints. Otherwise, a warm location which is free of drafts will be fine, particularly for parts which are only a few cm high.
If you're not using an enclosure, the part cooling fan should probably _not_ be used to print ABS. You should also be aware that ABS tends to generate more noticeable fumes than PLA (although this varies with product, and how sensitive you are).