jps, Just use a sheet of black cardstock from the newsagents. Cut to cover your tube + a bit to fold over the edge. Cut 2 or 3 holes in it, prob about 50 ~ 60 mm in diam. ( correct here if nec, guys). You can use the ap. mask to aid in focusing as well. When the objects come together, you're in focus....
They also cut down the brighter objects when trying to split doubles etc & as said, help when collimating.
I use 'blue tac' to hold in place & when storing away.
HTH.
L.