Now the connection of the tubes which will form the top of the pyramid:

First drill holes on 2 sides of the top ends of the tubes, as shown in the picture:

Mount two of the side tubes to the pyramid base so that they can touch themselves at the top.

Make the joining element out of a thin steel plate of 30 x 20 mm, bend it slightly in the mid, and cut the edges as shown on the picture.
Move the tubes so that they are touching themselves at the top, put the joining element on the inside of the two tubes. and mark the holes through the holes in the tubes using a permanent marker. Then drill the holes (i would recommend 4 mm, depends on the screws you want to use).

After drilling the holes, the joining element should look like this:

Mount the two tubes together using the joining element.
The result should look like this (seen from outside):

...and from the inside:

Repeat the last steps for the remaining two tubes.

Then the frame will be finished.