Okay, so this one is gonna be one of, if not the toughest, searches for me in the future once I transfer to my next apartment.
I've got several PL-L 18w lamps... two of them have seen heavy usage over the years, and the rest are replacement lamps.
Those two being used are inside of a slightly mangled Sadelite desk lamp, using two separate magnetic HPF R.S ballasts for PL-L 18w lamps.
Does anyone know of a PL-L 18w desk lamp that could fit a HPF Rapid Start ballast? Keep in mind, since these ballasts are magnetic HPF,
they are larger than the LPF versions (if they exist) and also larger than the more average PL-L electronic ballast... so the desk lamp
would need to have a large enough base to fit the ballast. I don't think I'll find one, but has anyone ever seen such desk lamp?
 
I may also have to create one myself, somehow.

It is important to know that the 10.5” 18w PL-L lamps, NOT the 9” ones are the ones that the 18w PL-L rapid start ballasts are intended to run. The 9 inch lamps will overheat the rapid start ballasts due to the low arc voltage drop. However, you might try using F20T12 trigger start fluorescent tube ballasts if you want to use 9” 18w PL-L lamps on a rapid start circuit. In addition, it is also good to know that F17T8 magnetic rapid start ballasts can run the 10.5” long 18w PL-L lamps.

That's some good info, since my Philips 18w lamp seems like it's more than likely 9", and my Sylvania PL-L lamps (which all of my replacement lamps are)
seems to be noticeably longer, likely 10.5", like you say. Even though there is slight flicker from the Philips lamp on the magnetic PL-L HPF R.S ballast,
the lamp seems to be operating fine with it. I've used that lamp with that ballast since 2019.
What I don't like... and I'll brief with this... is the fact that I have both ballasts in one Sadelite desk lamp, squished inside the base, right next
to each other. It's been this way since 2019, heavy usage seen. The ballasts both get hot, but not as hot as I would expect. That's surprising to me.
But that is why I bring up this topic. I want to find a desk lamp that enables me to operate one PL-L lamp with a full sized HPF Rapid Start ballast.
I do also have HPF Trigger Start ballasts, including cold temp versions, that may be able to operate the 9" 18w PL-L, but I want to stick with F15T12,
or F20T12 lamps for those particular ballasts.