Marudai is one type of loom that is used to create kumihimo. Maru (round) dai (stand) can be comfortably used to create maruhimo (round braids) and harahimo (flat braids) that have 36 or fewer strands. Cords with more than 36 strands can be created, but the loom will be crowded.
The first thing to consider when biulding a marudai is the height of the loom - Japanese looms are 16 inches (40 cm) high, and are intended to be used when kneeling. If you are going to be doing your braiding while sitting in a chair, you will need a taller loom. Most commercial providers sell Western looms at 26 inches (60 cm). There are other options - you cuold place the loom on top of something when working in a chair to bring the loom to the right level, or you could create two sets of legs - one short, one tall. It is advisable to have the legs removable anyway, and making an extra set is not a lot of work.
The second thing o consider is what type of wood to use in your loom. Traditional looms are made of maple, but any wood can be used.
There are three sections of the loom - the top or kagami (mirror), the legs,and the base. The mirror can 10 inches (25 cm) or 12 inches (30 cm) in diameter, with the centre hole 1.5 inches (4 cm) in diameter. Ideally, the mirror will be turned so that there is a slight impression around the centre hole (see illustration). This is not necessary, however. If you do not have access to a lathe, simply cut a hole in the centre.
The legs can be attatched in any way - whatever works for you. Most commercial marudai are threaded where the legs attatch to the mirror, and attached with a type of screw through the bottom. I have a loom whose legs are simply tight-fitting dowels.
The stand is simple square with slightly larger dimensions that the mirror. Decorative touches can be added like the loom in the illustration. I have also seen looms that have a slight depression in the stnad, and this is asthetically pleasing.
All edges should be sanded smooth - some form of varnish or sealant may be used, but you do not want the surface to be slippery. The finished surface should be smooth enough not to catch the threads, but give enough traction so that threads will stay where they are placed.