Soldering your turtle onto a piece of steel or brass isn't hard at all if you have a propane torch, some soldering flux, some soft solder (lead or tin) and a piece of fine grit sandpaper, a piece of steel or brass that is the same thickness as the depth of the dovetail in the barrel and a flat file.
Plain old lead or tin solder will be more than enough to hold your turtle onto the base for hundreds of years.
The solder flux should be the kind for steel, not the rosin core stuff used for electrical wiring.
The first thing to do is to cut the steel or brass base so that it fits the groove in the barrel.
This 5/8 dovetailed piece only has to be as wide as the width of the barrel where its dovetail breaks out on the side flats.
If you are working on a piece of stock for the base first file one of the edges so it has close to a 60 degree angle on it. Place the filed edge into the barrel dovetail and scribe a mark on the material (with a sharp nail) where the other end of the dovetail is at.
Use your flat file to file down the width of the material at a 60 degree angle until you just about reach the scribed width line.
Then slowly file more until the piece just starts to enter the barrels dovetail. A few swipes more of the file should produce a piece that could be lightly tapped into place.
Mark the needed width on the stock and use a hack saw to cut off a piece that is just slightly wider than what you think you need.
Use the sandpaper to lightly sand the top surface of the stock and the bottom of the turtle.
Apply a small amount of flux to the top surface and heat the piece gently with the propane torch.
Do this slowly and every little bit, touch the end of the solder to the surface and hold it there for a few seconds. DO NOT aim the torch at the solder and do not overheat the base trying to speed things up. That will only destroy the flux.
When the metal is at the right temperature the solder will melt. You might want to add just a little more solder to the top surface until it is covered. Then use a cotton rag to wipe off the excess. You now have a "tinned" base.
Use your flat file to make sure the bottom of the turtle is flat. Then apply a touch of flux to the bottom of the turtle.
Reheat the base slowly until the solder melts, then place the turtle in the center of the base and move it around until it is pointed in the right direction. You may want to aim the torch at the base every once and a while to keep the solder melted.
When the turtle is where you want it, hold it down with the end of a screw driver or something that won't be damaged by the heat (like your fingers).
Remove the heat and hold the turtle in place for at least 30 seconds. After this time the solder will be hard.
You can use the file and sandpaper to remove any solder that is exposed.
Use the file if needed to do a final fit to the barrels dovetail and tap the new sight in place.
After you've installed the sight, test fire the gun to see that the sight is in the correct place. If it isn't in the correct place you can drift it sideways until it is. Then you can file the small protruding edges of the sight base down to be flush with the sides of the barrel and you will have a professional looking turtle sight.
Have Fun!