The problem with couches are that their arms are fabric. So you can’t put say a coffee cup down on one. With an arm cup holder you can put your cup and other things like the remote on the cup holder.
Use a 4″ wide piece of lumber, make the following length cuts to make five parts,
Part 1- 3″
Part 2- 5 1/2″ (this piece is adjustable for your size chair. Make the length what you need to span the arm of your chair)
Part 3- 6″
Part 4- 4″
Part 5- 3″
Attach Part 1 and Part 3 to the underside of Part 2 with the pocket holes both facing in.
After the project is stained, seal it. This project is going to be exposed to moisture, so sealing it is important.
