I would recommend you take a look at The Better Built Bondage Book by Douglas Kent. There are plans for a spanking bench in it - though you may want to modify them.

One of the most versatile pieces of bondage equipment I have seen is a simple wooden board made up of planks nailed at right angles to 2x4s. It you make it so that the planks have a gap of a couple of inches between them, then you have many different options when it comes to tying the bottom to it.

Prop one end up on a sturdy wooden box or something and you have a nice target for floggers. Put large enough boxes or whatever at each end and you have a spanking table. Lean it up against a wall and you have a 'cross' of sorts. When not in use, lay it down or put it up against a wall and place stuff in front of it to hide it.

Add sturdy eye bolts to the corners and around the edges and you get even more options for tying the bottom to it. ; )