I don't have a .410 or any .410 shells so I can't tell you the exact length needed for a plug but it should be simple to figure out. I have a Model 12 in 12ga that needed a plug and I made as follows:
You need to cut a dowel rod that can fit down the magazine spring long enough to take up the space occupied by the capacity of the magazine to hold 3" shells minus the space needed for two 3" shells plus a little more to allow for insertion of the shells in the mag tube beyond the shell catch. I'd suggest + 1/2 to 3/4". Use actual measured shell length.
Because shot shell length is classified by the length of a fired case that includes the unfolded crimp (total length fired is 3" on a 3" shell), the length of a unfired case is shorter unless it uses a cap end rather than a folded crimp. The plug may rattle a bit or slide up and down the mag tube but has worked for me for over 40 years now.