With this adapter you can easy make a wooden x-bracing.

I have used it now for a shelf that wasnt very stable and it stabilizes it pretty good 

Per bracing you need 4 wall_inserts and 1 X_insert

Ive attached the fusion files so that you can easily adjust the size for any wood beam.

The standard parameters are: 

insertion_length 50mm
angle = 45 degrees (best to divert the forces the best)
width_bar = 48mm
height_bar = 24mm
thickness = 3mm
thickness_wall  = 5mm
screw_space = 10mm

in the slicer images you can see what which parameter is. I would add a little clearance if you dont like to sand.

The printing settings are:

2 wall at least (the more the stronger)
15% infill
0.2mm layer height 
enforce supports for first 60 layers (only for x part), if your slicer doesn't have that option you can draw the support as in the image.
i would recommend petg
orientation as in the pictures 
I would recommend PETG or ABS for strength but PLA should also work

the formula for calculation to calculate the length of the wooden bar you have to cut:
length_cut = ((L-2*wall_thickness/sin(angle)-width_bar/tan(angle)-(width_bar+2*thickness))/2

L is the length between the two columns where you put the bracing.