Followup rod sections should be slightly longer than needed. Score more than you need and lightly sand the rod to remove any plastic flare from the knife cuts, then separate the sections by snapping them apart. Square up one end.
Make a second rod section the same length as the template and use those for the ends of the string. Align the sanded sides of the rods as you tack glue them with liquid cement applied with a brush to the number and length of depth charges you need. Once you get the length you need, paint some liquid cement on the bottom side where they touch.
Let the assembly dry thoroughly, then use a sanding stick to shorten the middle rods to the length of the end ones. This takes longer for me to type than to do, but you end up with a tidy row of uniform depth charges. Works for me, at least.