Have one of those generic kids fort building kit consisting of rods and connecting balls? Are the structures falling over because they are too flimsy? These 90° corner braces in 1, 2, or 3 axis worked for my kids and they may work for your kids' fort as well (or for your fort… I won't judge)!

• Designed to use as little material as possible and print with no supports, so the center wall is intended to be 1 line-width thick. You should be able to print four of these with ~150g of filament.
• In the kit my kids have, the rod length is ~16-inches, and the diameter is approximately 9.3 mm.