How to make a "shelf" to place your chests sideways?

[Elvenkind]

It's so annoying, I've seen it on film on youtube dozens of times, but today when I tried to make a half-roof, then it would not, absolutely not stick to the wall, placing a foundation on top of a foundation, simply look weird, and one can't make half or quarter foundations. So what is the trick again? I just want a little shelf where I can place my loot both on the floor and on the "shelf". BUT HOW IS IT DONE?! I can't find the right words to google my way to it... Please help a frustrted fellow player!