Create SignBox from metal sheet - not specific X8 problem, rather signmaking problem

Good Afternoon to all

i describe the usual procedure that i use to make signboxes with VERTICAL edges (is 2 sided sign box -i show the one size- similar for the other side)

everything goes fine with VERTICAL EDGES

....when NO VERTICAL EDGES the doors of hell open

an example

if someone can help i will appreciate that

sorry for my English