I did a Google search on the subject and all I could find is one the Brits can get in Europe. I didn't see anything made here although there may be someone that has fabricated one. I would suggest getting some aluminum tread plate and make your own. I would have it hang from an upper step and have it lock over a lower step with a padlock that has a removable hasp that would come through holes drilled through the tread plate at the bottom so it is easy to get to and remove. You would want to put some rubber strips on the tread plate metal where it rubs the ladder. I doesn't have to be a heavy gauge metal and only has to cover 2 or 3 steps. Lowes or Home Depot may have enough in stock to do the job.
2014 Palomino Puma 25RS
2011 F-150 Super Crew XLT
Days camped in 2014 - 23
Camped 2015 - 47
Camped 2016 - 51 done - 6 more planned so far