Manual Prop for TT cover

Here you go. There are several examples of Lid Stays on this page that could easily be adapted for dust cover duty. 

The other side of the coin is lowered dust cover reflects vibrations emanating from the cartridge back at the cartridge thus enhancing a resonance that interferes with transduction. Listen both ways and decide for yourself. But when you do that experiment, remove the dust cover completely.

It seems like a large plastic dustcover would serve as an amplifier of acoustic feedback into the whole turntable. And that would have a more deleterious effect than any possible shielding of sound with a cover down. Of course, not all tables and hinged covers are the same. Try it with and without the cover at low and high volume and report back.