Print with thick walls for max. strength. I'm used 2mm thick walls
To assemble you will need:
8x M3x45mm bolts
4x M3x12 bolts
12x M3 nuts
A lot of M3 washers. I suggest a washer for each nut and bolt
Depending on the thickness of your flight controller stack, you will need 4 additional M3 nuts and bolts. You can either just attach your stack with M3x20 or something like that, but for better frame stiffness I suggest printing some standoffs and attaching everything with 4 additional M3x45 nuts through the top plate.