you need thèse kind of wood as you can ses on 4th picture and some tubes on 5 th picture.
i started to cut the wood with the size i needed, i have choosen 20 cm / guitare for the width and 80 cm for the height. before linking you have to put the tubes on the pièces of woods. i chose nail pressure. finaly you build your holder with à little overflowing of 10 cm and its done