Hi there, and welcome!
Your sketch gets the gist of it, however, with "stock" parts that would create a "one shot" (or at the very least a finicky semi-auto where the time you depress the trigger for dictates the amount of air dumped each shot) launcher.
wyz2285 modified his blowgun to only exhaust a small amount of air each time it was depressed. You can see his design in the
original thread. EDIT: removed incorrect link.
Also, that loading mechanism can work nicely. I haven't done a sliding cover in the sense you're talking about (or used a magnet to keep the steel BB from rolling, which certainly is nice), but I did use a drilled out compression fitting instead which I simply cranked down over the loading port to seal off the hole and keep the cover in place. (which I am having a hard time finding a picture of... have to get back to you there)
There are also a few more "succinct" ways to do semi-auto with a Q.E.V. and three way valve (simply use the three way to pilot and fill the chamber), and various autoloader techniques you can browse as well.
Good luck with your project, if you have any more questions feel free to post them here.
EDIT:

As you can see I simply drilled out the stop in the compression fitting to slide over the barrel. A quick twist locks it in place.