Take the top board and do the same thing as done on step(4) with the "L2" braces. Put glue on connection, the top board and heavy weight on the top of everything.

Here is the technique I used in order to hold them in place while drying:

Take a look at the cross created from the long rounded braces and the woofer braces.

Continue seal with RTV silicone also from the outside.
Wipe carefully with dry cloth!
You will have to color the MDF and the silicone layer will not let you.