When I made a mod for Quake1 "The Hangar"of the first Doom level,
there were a lot of sprites like the health bowls.
I made them in Quark6.4 in the map editor, by constructing a kind of skull with the brushes,
gave them a decent skin texture, and put them excact on the 0.0.0 grid.
Then I made some lights around them so it gave a dimensional shape.
There may be no walls around it, just the object itself.
When compiling the vis compiler will fail, because it reports a leak.
I put the skull.bsp in the entity list and in the items.qc I added a script
for a removal object like the health kit and compiled a new progs.dat
Then I could use the skull.bsp as an health kit in the Doom mod.