To compliment Ralph's mill backstop, here is one for your lathe chuck.
This is an extract from an email I sent to a good friend.
Find attached a C-o-C for a chuck backstop. You might need to find out if the morse taper can fit thru your chuck, if it does, then just use a length of bar to tap it back out from the rear of the spindle when you have finished. I have turned the tapered bit down before now, so it isn't as wide at the big end, just to get it to fit thru the chuck. With all my backstops, I just turn up a range of rod lengths and diameters and use what is required for the job.
I hope it can be understood.
Bogs