If you fish with a fixed-length-line rod, there is about an 98% chance you belong to one of two groups -
1) Folks who have lost a rod plug
2)Folks who will lose a rod plug
Fortunately, you can easily and cheaply make your own replacement plug. The best material I've found is foam from a cheap pair of flip flops.
Start off by cutting a block of foam, slightly wider than the diameter of your rod. You may want to use a brightly colored flip flop, ideally not one that is still in use by your significant other -
Trim your foam block to a length of 1 1/4 long
Make a slit sut on each corner of the block, about 1/2" down from the top of the block
Trim the corners on the bottom 3/4" of your block
Give it a test fit. You want a slightly snug fit. Do not try to force fit the plug, if it's too big, trim away a bit more of the foam
The foam makes a very functional rod plug. Maybe not so pretty, but by using a bright color it will be easy to find if you drop it.
Now that you have the hang of it, make an extra or two. Put a spare in fly vest/pack.