I made this design based on the Improved Anet A8 RepRap X-Axis Belt Tensioner by WedgeSD, which won't work with the stock Anet belt and won't fit into an IKEA Lack Table enclosure.
You only need :
1 M3x30 screw
2 M3 nuts
5 M5 washers instead of a pulley
no belt upgrade needed
The parts are designed to fit exactly in place, but I needed to do some sanding and lubrication (PTFE) to get it to work.
With this upgrade, the printer should fit into an IKEA Lack table enclosure.
EDIT: After a few months of use, I saw that the washers had damaged the axis so much, that shortly afterwards it would have broken, possibly resulting in severe damage. So please: ONLY USE THE AXIS.STL TEMPORARILY if at all. I switched mine to a cylindric piece of a screw (without thread) that I cut to the right length using my Dremel. It's really safer if you use a metal axis. And smoother, since it has less friction.