If your Dog has a comfy harness already and you don't want a new LED specific one you can convert it by adding on a TPU collar twisted into a figure 8. It loops through the handle on the top of the harness and hugs their sides creating a super bright figure 8 ring across their back and sides.
This works very well for dogs with thick necks or dogs with a lot of fluff where LED collars can be difficult to see.
One size fits all Harnesses but you can cut it shorter if you have a particularly small dog.