1) Remove your files for smaller explosions. These use the same txt files for the seeker crosshair so they'd overwrite the crosshair if you keep them
(I did include smaller explosions in the seeker crosshair files though so not much will change. They're not the same smaller explosions so if you don't like them let me know and I'll change it to the ones you use)

2) download this and put the folder in custom https://www.dropbox.com/sh/vlw3eyvs455flqb/AADsk9rlkLrAuwedAAncKX9ea?dl=0
3) Restart TF2.
4) use cl_crosshair_file "" in console
You'll now have seeker crosshair. You can recolor and resize them just like regular in-game crosshairs with cl_crosshair_scale etc