This is a Fisher-Price Contrux plate that fits a square made of 4 long construction bars or 8 medium length bars. I don't believe this part exists in the Construx catalog, so 3D printing to the rescue!
 

If your printer is calibrated it should snap in and hold itself in place. Designed for 0.2 layer heights. PLA works fine. I included a STEP file to make it easy to remix.