Trimming the top brace on a 40 breeder...

[Peace River]

This turned out to be a lot easier than I expected. I was able to cut into the lip by taking a jigsaw blade by hand and just working it for a few seconds. I made the lengthwise cut with a utility knife. I scored it once at first, then folded it back and hit it a second time with the blade, which took it right off.

It might have taken me 5 minutes to make the cut, then another 5 minutes worth of sanding after that.

I am glad that it worked out for you! As you found out, the trim is pretty soft plastic and can be cut with a utility knife - I have done this before. Your cut doesn't seem to be particularly structural so it shouldn't be an issue. To be on the safe side, one thing that I have done after tank mods like yours is to get tank measurements at three places across the top of the tank (and take baseline pictures with a phone of the measuring tape on the glass) and then recheck the measurements every few months to make sure that there is no visible bowing. I know it sounds like overkill, but it may help prevent a blow out for older or modified tanks if bowing is detected. Just a thought!