I use painting wands to hold my rockets horizontally when spray painting.
The wand is a 1/2" dowel shoved into a used 18mm engine casing.
This is too small a diameter to fit this larger rocket engine mount.
I wrapped some corrugated cardboard around the casing and closed it up with tape. To fit the 29mm diameter mount, the cardboard is just over a full wrap around the casing. This wider diameter gives a good friction fit. 
With the dowel wand slid into the mount,
Some masking tape is placed sticky side out inside the main body tube.
A rolled up paper towel is press into the area around the mount and up against the inside wall of the main tube.
Any open areas are covered with additional pieces of masking tape.
Now the rocket can be spray painted without getting any paint inside the low end.
Curious why you want to keep the paint off the tail centering ring... Keeping the inside of the motor mount clean makes sense, but normally I paint everything else (albeit not necessarily all that carefully).
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