A jig to mark the middle of a strip of duct tape.

1. Print and attach a top and bottom part. Make sure the arrows point in the same direction.
2. Pull out a strip of duct tape.
3. Thread the tape through the jig, with the glue side facing towards the spike.
4. Lightly pull the tape downwards, so it snugly fits between the sides of the jig.
5. Firmly press down on the spike side and pull the tape through.

This will leave a mark in the glue down the middle of the tape.

See https://youtu.be/_HH1vLw6Jr0 for more details. This was tested with rolls of grey and black Lockport rolls. For some reason, their gray tape rolls are a couple of mm thinner than their black ones - measure the width of your tape before using.