Your cat will not like the scratching post if it is not stable, so we will use mortar to make an heavy post foot. I used 4kg of mortar, which is enough for my 3.5kg cat; if yours is heavier, you might want to use more.

Your can get a lot of information in this concrete class; it gives some hints to give a custom finish to the molded parts, in case you do not like the white aspect of this DIY. We will use mortar instead of concrete, because it is easier to obtain a smooth surface using mortar (concrete contains gravels).

You will need a big bowl of about 24 cm side length (or diameter) as a mold (its size and shape may vary depending your taste and supplies). As you are going to pierce a hole in it (for the threaded rod), the best choice is a cheap plastic basin. I found a square one with rounded edges, which gave a nice shape to my scratching post foot.

The foot must contain a captive nut on which the threaded rod will be screwed on. This threaded rod is the backbone of the post, and gives its sturdiness to the cardboard tower. So once you have found the proper basin, you will have to drill a 8mm hole in its center.

Pouring the mortar in the basin with the whole rod inserted is not going to be convenient, so you will have to cut a small part of the rod (about 4cm long) as temporary guide.

It is better to lube this rod to help extracting it once the mortar is set (I did not lubricate it, but was nevertheless able to extract it). Screw a first nut at one of its extremity, then another one 1 cm farther. Insert this rod into the hole, so that the nuts are inside the basin, then fix the rod by adding a washer and a third nut on the other side. Make sure there is enough space to add a fourth nut on top of this last one, as these two nuts will be necessary to extract this temporary guide. It is also important for the guide to stay straight, so ensure you have some supports on which to place the basin so that the guide is not pushed away.