So Singer decided that the Promise should have a momentary switch to control forwards/backwards movement. I have no clue why they thought this was appropriate - it often, especially when working with tough materials like webbing, leads to me trying to do four things at once: holding down the thread, guiding the materials, holding reverse, and turning the hand-wheel.

Hence, a latch, to make the switch into a proper toggle. But a deadbolt-style latch, so I can slide the device out of the way in case I want a momentary switch.

You'll need:
A flexible peice of metal about 12" long (I used an old hacksaw blade)
A grinder (bench or Dremel)
Hot-melt adhesive
A propane torch

Step 1: Make the Metal Bits

I'm assuming you'll have a different sewing machine than me, so I'm not providing measurements. It should be pretty obvious how long to make each piece.

Before separating the hacksaw blade into pieces, you'll want to grind off the teeth. Try to keep the width of the metal strip consistent, especially for the first 5 inches or so (which will become the sliding bolt of the deadbolt), by grinding off the same amount of material along its whole length. You can monitor your grinding process by watching how big the flat, shiny spots on the teeth are.
Once you've ground to depth, you'll probably need to remove the through-thickness variation - "waviness" - they introduce around the teeth so that they cut a kerf slightly wider than the rest of the blade.

Then, break the metal into peices with pliers, as shown, and grind to the profiles illustrated. You'll want to carefully deburr everything to make it smooth.

Two of the pieces, the bolt and the top restraint, need a bend in them. Heat up the area of the bend with the propane torch and then bend them 90 degrees with pliers.

I sanded most of the paint off of these pieces to make them prettier, but it wasn't really required.

Step 2: Assembly

Roughen the place where you will glue the metal bits with a bit of coarse sandpaper.
Clamp a vice-grips, or similar heavy pair of pliers, to the toggle switch, to hold it in the "down" position.
"Tack" the bolt in the position you want it to be to hold the toggle switch down, using a wee bit of hot glue.
Holding the top restraint with a pair of narrow pliers, warm it up using the propane torch, apply some hot glue along its length, and glue it adjacent to the bolt, in the position shown.
Warm up the bottom restrain, apply glue, and glue it adjacent to the bolt. Note the caption on the picture - I screwed up a bit in the assembly. It's ok, though...
Warm up the hold-down ties, apply glue, and glue them over the bolt. Make sure they are far enough back from the toggle switch that the bolt can flex out of the way of said toggle switch.
Pull or push on the bolt to free it from its hot glue tacks.
Warming up only as many of these glue joints at once as is required, shift everything as required, to make the operation tight yet smooth.

Step 3: Use

With the bolt in the active position, you can lift it out of the way to easily switch between forwards and backwards with a single hand (but, admittedly, two or three fingers).

Or, you can slide the bolt out of the way and use the momentary switch as Singer intended.