If you look through the focuser and you can see what's behind the scope just outside of the primary mirror (e.g. grass), then you need a baffle behind the primary.
That sounds like a good idea, but it seriously reduces the air flow around the primary.
Instead, I accomplished the same thing with a large black cloth on the ground under the scope.
I found a large black tablecloth (10' x 8') for the purpose at a 99 cent store. Yes, I can see the ground under the scope in reflection in the secondary mirror, but the ground is jet black at night when you look down in past the primary mirror.
I also use a light baffle extension attached to the upper tube assembly opposite the focuser so no light can enter the bottom of the focuser from over the other side of the tube.
Light scatter suppression is accomplished by using:
--flat black secondary vanes
--flat black secondary holder
--flat black interior to UTA
--flat black interior surface to UTA rings.
--flat black pipe insulation on the poles**
--flat black interior to rocker box
--flat black interior to mirror box
--velvet on inside of UTA opposite focuser
--two circular baffles in mirror box
--velvet on inside of mirror box
--flat black top to mirror box
--flat black bottom to UTA lower ring
--flat back hole in focuser board where drawtube moves through the board
--flat black inside to drawtube
--flat black inside surface to focuser board that faces the secondary
--any/all nuts or bolts inside the OTA flat blackened--no shiny surface anywhere.
--black lycra shroud with matte side facing in
--upper tube light shield protruding 1" up from UTA**
Everything together yields excellent contrast. A couple of those were my idea (**), the rest were Rob Teeter's.
As for Seronik's idea, light can enter the bottom of the focuser from more than the axial line, so a plug with a center hole is insufficient.
You need to remove anything in the focuser and look in the focuser at an angle. If you can see past the top of the UTA, you need a light shield opposite the focuser.
Tom Dey is right about a domed observatory--I had a portable dome several years ago, but I sold it because I couldn't see the sky other than through the slit.
I was constantly walking in and out of the observatory just to see the sky. A roll-off-roof observatory eliminates that issue, but also doesn't offer
the same degree of ambient light suppression.
it's true, though--optimizing light suppression in the scope is only part of the equation. Optimizing light suppression in your environment is the rest:
Black cloth over the head and eyepiece, a hood, an observatory, or a light shield bucket around the focuser--whatever works.