This thread brings back fond memories!
My father made my first "real" slingshot with the fork made from two pieces of 3/8" cobber tubing brazed together. The rubber cut from red inner tube (Black tubes were Butan rubber and not elastic enough) His plan was to use it to keep stray dogs out of mum's garden, but I quickly took possesion of it and carried it everywhere I went. Ammo was pebbles or steel "wads" - the leftover from hole punching at the local farm machinery plant. Those days I was always wearing an anorak (my mum sewed them for us) and the extra large breast pocket would hold a large ammo supply.
For some reasons I always shot the slingshot holding the fork with my right hand, whereas I shoot the bow holding it in my left hand. A couple of years back I took up slingshot shooting again, but can't seem to hit anything when I trying aiming using the top of the fork, like the eksperts (whose skills I envy greatly!) recommend. I think part of the problem is my right arm is not as "stable" as my left arm. When letting go of the sling my hand will jerk and spoil the shot. More practice is certainly called for.
This is one of several designs I've tried so far: