Here is a delightful puzzle that isn't too difficult.

There are 6 pieces, when assembled it will create a 2 layer square, 3 pieces by 3 pieces.

Version 4
I have reworked this puzzle several times for strength of the locking tabs

Included is a tolerance test if you would like to check the fit before printing the entire puzzle. Please make sure that you have your printing settings dialed in for elephant foot compensation.

There is a solution model that is a single solid block that shows the completed puzzle. The numbers on the solution indicate the order in which the pieces should be assembled.