Step 1: Alternate Prior Designs
Originally we would put shiny stickers on her back, to help distinguish her from the grass that she would be roaming around in. These would stay on quite well, and not interfere with her movement at all. She still managed to sneak away quite often however.
Making a little tactical suit for your tortoise is super tricky, especially because they have retractable limbs and don't like sitting in your lap while you try to get measurements. In the past I have made harnesses from cardboard (bad), duct tape (alright), bendable wire (alright). In general harnesses are super difficult to create, put on, and take off the tortoise.
Those soft, velcro cord organizers (http://cableorganizer.com/wire-wrap/) were the best things that I have seen for quickly constructing a nice harness. Generally harnesses are OK for a temporary situation, but inevitably the tortoise will find some way out of the harness or get herself caught.
Making your tortoise appear physically taller can help to locate it, but unfortunaltey prevents the poor creature from crawling underneath things which is probably one of its favorite activities.
The most failed attempt at tortoise visibility was attaching a bouquet of balloons to her. Mostly she did not like wearing the rudimentary harness to which the balloons attached, but the other problem was that when the wind blew, it could actually blow the balloons around and tip the poor tortoise over. I was kind of cool seeing the bundle of balloons navigate around the apartment and park. Here is a video dramatization of what could happen with such a system: