im,
As has been said, you need a regular magazine cap and a standard length magazine spring. You can get both from Brownells. You can get the magazine spring retainer, or leave it off, as you prefer. A reasonably healthy person should be able to contain the magazine cap when it's removed with no trouble, even with no magazine spring retainer in place. As with so many spring loaded gun parts, controlling the parts during disassembly is necessary.
The newer magazine caps are a 'one size fits all' design with both the necessary dimples around the rim of the cap for the detent, and the 'teeth' inside the cap for the post magazine tube dimples plastic ratchet. But the thing to be sure of in buying a magazine cap for an 870 with a detent in the barrel ring like you have is that it does in fact have the dimples around the rim of the magazine cap where it contacts the barrel ring.
fwiw,
lpl