Mounting batroom bar light above vanity mirror with ceiling box?

I want to mount a bathroom bar light above the vanity in my bathroom.
This is old work. I already have the electrical work done and the wires are ready the issue is that I don't have a junction box in the wall above the mirror.
I was going to get an old work ceiling box from home depot and mount it in the wall.
Is this the correct item to get?
Also how much weight can it support? I guess there will be some contact force with the wall that will aid in the support.
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Some strip lights can accept a cable directly into the back plate. For those that cannot, use a cut in switch box, mounted horizontally. The box can hold six pounds, however this type of fixture is usually fastened to the wall in addition to any box attachments.

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