Just made myself one last weekend - about $45 in parts.
I also checked at Home Depot and they didn't have many of the parts necessary.
I never could find a 'test plug' like shown on the websites.
I ended up with a fully PVC unit that was threaded on the end and screwed into the plunger.
I called around and Berry's has a stainless one for $100.
Another bait shop I called (Maple Ridge?) said $115 for stainless.
Have been hoping to try it out but the tides have been wrong all week.
Tomorrow looks promising.
Check YouTube for videos on how to operate the pump.
They are frequently called "Slurp guns".
Here's the plug that I used.
Note that the adjustment is inside the larger pipe.
I left the cap on the top of the larger tube unglued so that I could remove it for adjustments.