My troop as a youth held a troop-wide Eagle CoH at the same time every year, and alumni planned around coming to honor the new Eagles. It’s been more than a few decades, since I’ve been living pretty far away, so my data is fairly hazy.
I recall that we typically hosted it outdoors at a local park in August, so that we could keep costs down and had the best chance of alumni who were home from college dropping in, as well as having any scouts who were traveling (either with the troop or their families) back since it was shortly before school started again. We generally had refreshments of one sort or another, as well as local “dignitaries” who were willing to come out for our Eagle Court of Honor. I don’t recall it having an explicit fundraising aspect, but I would be surprised if it didn’t serve in that role.
I agree with @Qwazse’s thoughts that having alumni come in to talk to the scouts is a great opportunity. In particular, I like asking them to speak about how having been a scout has influenced their “non-Scouting” lives: merit badges that sparked an interest, how they made decisions about what to do in school or work, particularly when the speaker was surprised at how something they did or learned in Scouting influenced a part of their life. I’m a big believer in showing youth how their early decisions can help shape their futures, even when they themselves can’t see it just yet.