since it sucks as-is, maybe try to find something that fits the INSIDE of a dart and use the thing to fire darts like it did the arrows? sleeve the inside of the dart with a soda straw might help, you might even leave some extra straw hanging out to have a longer "barrel" push.
either that, or cut the little flappy end off a whoopie cushion and make farting noises with it.
I say, "POLLISH THAT TURD!" and pwn at the next walk-in-closet nerf war.
they are made to shoot water and depend on the water to lubricate the seal. you might want to lube it somehow, to make it move easier and hold up a little longer. (don't want you to go and waste all that money)
I picked up a smaller douchy version of those things, maybe a third the length and with a tip shaped like a crayola. pretty much seized up just dry pumping it. one of these days it will run through the bandsaw for a postmortem.
P.S. I scratch my asscrack with a set of screwdrivers EXACTLY like that. in fact, you better sniff them and make sure their yours.
have you got access to a propane torch? I use torches to help with woodwork. you just roast the wood and scrape off the charcoal. pretty much like burnt toast.
try not to get too carried away with the heat, it causes any cracks in the wood to expand if the heat gets too deep.
it helps a bunch with sanding too, you just burn the surface and hit it with a sanding block or sanding sponge.
you could drill a hole for the thumb and wobble the drill to get it a little bigger, then burn it over and over to get it big enough.
otherwise take an old screwdriver and use it like a chisel. a hammer makes just about anything work like a chisel. torching the wood after will clean it up fine.
early crossbows (real crossbows) used a lever and a dowel pin to trigger the crossbow. the string was in a groove over the hole for the dowelpin (like your hole in the extra block) the handle was tied to the stock and squeezing it up against the stock pushed the dowel up the groove and forced the string up out of the groove.
you could cut a groove in the wood below where the notch on your plunger sits. the wire could be bent into a rectangle and the ends could be bent down like a little handle. the dowel hole would be drilled up through the stock and hit the middle of your groove is sawn. drill a hole in your dowel tip for the tiny handle on the wire.
the dowel would be long enough to stick out the bottom of the stock. you could either push the dowel directly or use a handle like the crossbows used.
as a side note, the notch and dowel setup is pretty darn strong. a friend made a real crossbow completely out of hardwood and the string, no metal at all. a stray shot bounced off a forklift, a steel beam and then went through a 12 pack of soda, lengthwise! it could easily kill someone.