There was a thread about not being able to find an egg container for large eggs.
This may work for you...
I wanted to come up with a simple and cheap solution, so here it is.
coat the cardboard containers your eggs come in with rice glue.
I made my own the other day but this morning i found a "recipe" on the net
and that is almost exactly what I did..
After 3 coatings my container (half of the dozen version, so missing some support) has a relatively stiff "plasticky" feel and can support at least 3 of my 4 kg bricks ( 26 lbs total) . I could not balance the third one on top...
Better to do it in small layers so as not to put the original container out of shape.
(it will still absorb water so you may need a plastic bag around it. Test before using it...)
Mostly the idea is to "stiffen" the original container, however you want to do it....