I bought one last year and the largest I could go was an opening of 13x9 ". And the door could only go on the stove/fridge side of the boler
without having to relocate too many things. In my case I had to relocate the plastic cover flap to my plug in power cord. For me this was the biggest opening I could go without running into tail light
wiring and the curve of the boler
. I found mine on ebay from a guy who sells salvaged rv parts. Good luck.