First buy a pizza. You will want to use the box lid and one short side. The short side bent to approx. 20 degrees will align nicely with the rear bolt hole. Cut the size/shape you need to mount the top box mounting plate to the steel mounting plate that will be a permanent fixture on the bike in place of the passenger seat.
1/8th steel plate should be adequate for the steel mounting plate and easy enough to work with. I had one cut out and bent at a fabrication shop for $2o.
I made sure the cardboard cut out was precisely what I needed and knew where the mounting holes would be to match up to the bike. Measure measure and measure again. After the plate was made up I only needed to mount it on the bike and take some more measurements for holes to mount the top box. What I did do was drill 3 sets of holes an inch apart so I would have the option of moving the plastic box mounting plate fore or aft to make the fit more optimal for Lynda when she was riding.
Lots of measuring meant I did it right the first time with no goofs and all the drilling was away from the bike on a work bench or solid stool with the metal plate clamped in place.
After a test fit of all components I I removed all the fabed stuff and cleaned/painted it with tremclad. Assembly was with all metric hardware to match the bike. Overall it was a very satisfying project.
The top box in the advertisement should work well.. Keep in mind that the large reflective red area will likely not reflect worth sh*t. It just looks good in daylight. However the box has a nice flat strip running around it for a place to add real reflective tape. Also there is a nice flat spot on the rear side of the box too.