Rationale:

I wanted to be able to use https://www.prusaprinters.org/prints/26557-wooden-train-brio-ikea-to-duplo-adapters-mini on Duplo build plates, but the length of the various Brio and Melissa and Doug tracks doesn't correspond to the grid on Duplo.

I started off with a Duplo brick generator and modelled a part that suited my purposes.

Requires:

M3x20mm socket head screw (such as included in the spare parts with Prusa MK3S)

M3 hex nut (such as included in the spare parts with Prusa MK3S)

Printing Notes:

I printed these with support. 

Assembly:

• Remove support material
• Insert captive nut into holder in “Top Brick” (hold in place with a finger)
• Lay head of socket screw in feature on the top of the “Bottom Brick”
• Slide “Top Brick” onto rails of “Bottom Brick” and engage socket head screw with captive nut.
• Tighten to desired position using a hex key.