I designed these brackets to help in the rapid construction of RC airplanes + boats. I use coroplast for rudders, motor mounts, and decks, and using these brackets I can simply zip-tie those pieces at 90º angles to each other, with a decent amount of strength.

I've currently got three variants: 5x5x5cm, 4x5x7cm, and another 4x5x7 with thicker walls. If you have an Onshape account, you can customize the parametric model to your heart's delight here.

You can see photos of my particular use case (RC airboat) above, but these are generically useful to me for a wide variety of attachment purposes. Hopefully they're useful to you as well!