Sometimes a quick improvised tool or jig can really help to make a job easier.
Here's one that took about 2 minutes to make:

Trimming a cockpit coaming rim to the right width is pretty important especially if you already have a sprayskirt on hand - a too-wide rim/lip will make it tough to get the skirt on; too narrow and the skirt will pop off too easily.
Previously I'd used a marking gauge of some sort and then combined power and hand work to get the coaming to the final width.
The cutter is a diamond wheel (cheap from China on eBay) which works well on glass/carbon rims. You can get saw blades which might be a better bet on a wood laminate rim. ( I find most wood rims are too thick to grab the sprayskirt well, so I always use a glass/carbon coaming nowadays.)

One refinement: Notice the piece of white plastic tubing/hose slid over the collet collar. On most of the Dremel-type tools, that collar is knurled (steel) and will mark your workpiece if it touches it. You can see that this tool needs a new piece of tubing since the steel is starting to show.

On a keyhole-type coaming I can't use this tool at the wider sections of the rim, but it's still useful for the rest of the job.