From woof:
"Frankly anyone who has to install a battery should be going to the larger 51R now."

Is trimming the battery tray to accept the larger battery a big deal?

 

It's debatable. A cut tray still protects the sheet metal from abrasion and provides some lateral support for the base of the battery, but won't hold much acid if the battery leaks. If your battery never leaks (bubbles-over due to rapid charging) a cut tray is fine. Unfortunately leakage can happen, that's why the battery tray is tub-shaped and made of acid-safe plastic.
The earlier version (2015-2017?) of the battery tray (31521-T5A-000) fits the 51R battery and drops right in to later Fits. It's presently about $20 on ebay.
The later 151R-only tray is 31521-T9P-000, just so you know what to avoid.

 

I never saw the cut down tray as being an issue - many people have done it. When I did it I was more concerned about the small clearance between the new battery and the large heat sink a fraction of an inch from the end of the battery. I made sure I had the battery moved over as much as possible to maximize this gap and I made sure my battery was firmly fastened down so that it wouldn't shift around. It's been several years now and no problems of any kind.

And yes, Honda should have put a 51R battery in this car right from the factory.

 

It's not difficult to do if that's what you are asking. You just need a knife or a shears.

I cut my tray down and installed the bigger battery, but I didn't like how the 51R was jammed in there, so I went back to the 151R.