First of all you have to cut the cap off on each side rail. Lightly sand any ridge made by the cutting. Measure your dog so the new side bars will extend about one and one half inch past the back of the front shoulder blade.
Purchase a small container of PVC cement clear about $5.00. Also buy 2 - 3/4” PVC caps, 2 unions and a small section of ¾” PVC pipe about a few foot or less if you can, all in schedule 40 PVC.
The unions will be glued on the end of side rails that you cut off. Cut the new ¾” pipe the length you need, sand any rough edges, and glue it into the union. Finally glue the caps on and put the straps back on, allow room for the side reflector tees. They should be mounted 2” behind the new front cap.
All the materials should cost less than $10.00. Let it sit to cure for a few hours before using. If you ask any hardware store employee, in the plumbing section, they can easily walk you through the process.