Yes, and no. I've seen them with both flat springs and coils. Interestingly, while having some time to kill one afternoon this week I tried my hand at this very task. I used 1/2X1/2 inch stock and a coil spring. Mucked it up in the end but I believe it could be fairly easily done by someone with patience and a steady hand.

Milled the rough shape from the bar stock. The "standing leaf" was left double thickness at first. The recess for the (single) folding leaf was milled out of the front of this leaf with a 1/4 inch end mill - in from the front then in from the top of the standing leaf to clean-up and square the notch. I then drilled a hole to just fit the spring straight down into the bottom of this notch. The sides were then cut down to form the "hinge." I then cut a rough leaf from flat stock so it just fit the recess, clamped it together and drilled the hinge pin hole through the bolsters on the standing leaf and the tail of the folding leaf at the same time. The spring was cut to fit the recess drilled for it while putting (slight) pressure on the folding leaf in both positions. The thickness of the base only allows for about one coil worth of compression in the spring so go slow! The front edge of the folding leaf will need to be radiused to ease it over the spring when folding up or down but don't go too far or it will be loose in both positions. You want a FLAT contact with the spring as large as practicle.

In the end I made a slip of the file, which ruined my proto-type sight. Alas, the recent heat wave, my big fingers and lack of sleep of late aren't so conducive to such fine work!!! It'll take patience and a light touch my friend. In the end I doubt such one-off sights can be marketed cheaper than similar mass produced ones can be bought. Check out Jeff's Outfitters. The link is in the supply thread at the top of this section. Best of luck should you decide to proceed!!!