It's fairly easy to make an accurate template -- in fact more accurate than a template you might get from someone else, because it will be made precisely for your trailer. It's an old yacht fitters technique.
1) Take a piece of cardboard or thin sheet material and trim it to be an interim template of fairly close fit.
2) Secure it in position with tape or whatever. I either use packing tape or duct tape, normally.
3) Make some strips of thin cardboard about 1/2 inch wide and cut to a point at one end.
4) Tape strips to the interim template radially with the points touching the shell of the Boler
5) Remove the interim template and place on another piece of material designated to be the final template.
6) Mark dots at each point and connect the dots with a line.
7) Cut at the line.
8) You have just made an accurate template tailored to your own Boler.
Note: For one-shot use, in step 5) just place the interim template on the material for the new shelf and do the marking. Then do steps 6) and 7) and your new shelf is made.