Okay, you've got a classic electrical challenge: securely mounting a potentially heavy, cantilevered sconce without creating major wall damage. Let's break down your options and concerns with the Raco 393 and explore alternatives.

Understanding the Problem Load & Leverage

You're right to be concerned about the cantilevered weight. Those long-armed fixtures put a significant torque on the mounting box. A standard electrical box secured only to the drywall is almost guaranteed to fail over time, leading to sagging or complete pull-out.

Obstruction

The primary question, as you identified, is if the old work brace of the ceiling fan box can even fit past the 2x4 (or 2x6) wall stud inside the wall cavity. Even if it technically fits, maneuvering it into position can be difficult.

Rotation

The issue of the box rotating is crucial. Even if the brace is secure against the stud, the box itself can still rotate within the hole. This will lead to the gap you described.

Addressing Your Concerns with the Raco 393 Pancake Fan Box Will it Fit? This is the critical first step. Unfortunately, the answer is most likely NO. Here's why: Stud Depth

A standard 2x4 is actually 3.5 inches deep. While the Raco 393 is a "pancake" box, the old-work brace needs enough clearance to slide past the stud. The brace arms themselves have a width, and they spread out. The chances of all the metal parts of the bracket and the brace's thickness clearing the stud is slim.

Maneuverability

Even if the brace arms are narrow enough to slide past the stud, you'll likely struggle to manipulate the brace and box into the correct position behind the drywall.

Verification

If you are able to get inside the wall, you can measure the inside depth of the cavity, and the width of the 4" opening. This can give a definitive answer as to whether you can do it.

Preventing Rotation

Even if you get the Raco 393 installed, preventing rotation will be extremely difficult. Fan boxes are designed for a downward load, not a sideways cantilevered force. The friction of the brace against the stud might provide some initial resistance, but it's not a reliable, long-term solution.

Code Compliance

Using a ceiling fan box in a wall might not be code-compliant in your area. Check with your local electrical inspector. While it might seem like a good idea, it's typically not designed for that purpose.

Better Alternatives for Sconce Mounting

Here are more robust and reliable ways to mount those sconces, in order of increasing complexity:1. The Best Solution (If Possible): Mount Directly to a Stud This is the strongest and most secure option. Locate the Stud

Use a stud finder to accurately locate the center of the stud behind the drywall.

Cut and Modify

Carefully cut a smaller hole in the drywall (just large enough to access the stud).

Attach a Wood Block

Cut a piece of wood (ideally the same thickness as the stud) and screw it securely to the stud using long wood screws. This provides a solid, flat surface for mounting the electrical box.

Install a Standard Wall Box

Attach a standard plastic or metal electrical box to the wood block using appropriate screws.

Re-attach the cut piece

Cut 1/2" x 1/2" wood pieces and glue them behind the drywall cut out. Use blue tape to keep the drywall in place. Once the glue has dried you can install the electrical fixture.

Minor Spackle and Paint

You'll need to spackle around the smaller hole and touch up the paint.2.

Using an "Old Work" Box with Enhanced Support This is a better approach than trying to force a fan box into a wall. Heavy-Duty Old Work Box

Look for an old-work electrical box specifically designed for heavier fixtures (check their load rating). Some feature metal wings that clamp tightly against the back of the drywall.3.

Opening the Wall (Your Initial Concern) While it's more work, opening the wall is the most reliable way to ensure a secure installation, especially for heavier, cantilevered fixtures. Cut Access

Carefully cut a larger rectangular section of drywall between two studs.

Add Blocking

Install a horizontal piece of lumber (blocking) between the two studs, creating a solid mounting point. Secure the blocking with screws driven into the studs from both sides.

Install Box

Mount a standard electrical box to the blocking.

Patch Drywall

Replace the drywall section, tape, mud, sand, and paint.

Additional Tips Consider the Wire

Ensure there is enough wire length in the wall cavity to make your connections safely within the electrical box.

Grounding

Properly ground the fixture. Ensure the electrical box is grounded (if metal) and connect the fixture's ground wire.

Safety First

Always turn off the power at the circuit breaker before working with electrical wiring.

Consult an Electrician

If you're unsure about any part of the process, it's always best to consult a qualified electrician.

Recommendation

Start by carefully assessing the weight and design of the sconces. If they're relatively light, a high-quality* old-work box with secure clamping wings

might work, but only if you can verify adequate support. Otherwise, your best bet is to mount directly to a stud or open the wall and install blocking. A small amount of patching and painting is a small price to pay for the peace of mind knowing that the fixtures are securely mounted and safe. Trying to shoehorn a ceiling fan box into the wall is likely to lead to frustration and a potentially unsafe installation. Flag for review