I placed the box bottom in the middle area so the solid end was up. Next, I placed a piece of cardboard cut to fit snugly into the middle area onto the box bottom. Finally, I duct taped the piece of cardboard around the edges to hold it in place and to prevent leakage into the bottom of the table.
You can easily see the difference in in the children's operations with the false bottom in place. On the left, where there is no false bottom, the child has to stretch are far as she can to even get a cup. She gets the cup, but cannot reach the pellets. The child on the right, where there is a false bottom, stretches, but he is stretching to get the pellets.
For the cardboard tube, I had to cut an oval in the roof so the tube would be straight up and down. If I just cut a circle the circumference of the tube, the tube would enter the roof with an incline perpendicular to the roof. If I tried to force the tube to be straight, I would rip the cardboard.
On the inside, I duct taped the tube into a corner for stability. Notice, also, I cut a notch in the bottom of the tube so the pellets could flow out freely when poured down the tube. I learned the hard way that the tube has to be several inches off the false bottom otherwise some of the objects the children put down the tube cannot be extracted. (I say I learned the hard way because I had to destroy one false bottom to get a bottle out of the tube.)
The main difference was the added focus for play around the cardboard tube. That included pouring pellets into the hole.
Modifications create a whole host of new possibilities for play. In this case, they added great play value as the children appropriated the novel spaces and elements.