Please let me emphasize that I'm not sure about ANY of the following info, and you would do well to try any of them on a sample piece the same way you would do to your project, so you don't ruin it if something doesn't work.  I have been a chronic reader for fifty years, and have absorbed all kinds of information that I wasn't really looking for, thus didn't pay a lot of attention, but some of it stuck just the same.  Some of the possibly useful (and useless) info that I have acquired may have been mixed with other info, making it totally useless for anything.  My apologies in advance if any of this isn't correct.

Over 20 years ago, I read something where a person advised adding something like talc to a liquid gloss to make it matte.  I don't know how much, but I would start with just a tiny amount.  And I don't know what something like talc would do to an otheriwse impervious finish.

In oil painting (I think), I believe I read that if the artist wants a matte finish on a painting, they have to let the piece dry very thoroughly, then apply glossy varnish, and when that is dry they apply a matte varnish.  If you're going to do that, I would attempt to try to find a matte varnish in the same brand as the gloss, so there would be no conflict of ingredients.  And, if you can find a matte in marine varnish (I believe that it does exist), you might contact the mfgr and ask questions.  After all, if you find something that seems to work well, you want to know about any possible pitfalls.

If all else fails, and you put several layers of your glossy varnish on your project, you might be able to get some very (VERY) fine sandpaper and go over it very lightly to produce a matte effect.  This is pure conjecture.

Sue, who has no DELETE button attached to her brain

Having had some bad experiences with matte spray cans (clear coating), I'd like to say don't use a spray can if you can help it. heh I've a handful of test pieces here that eventually turned sticky in this humidity and have collected ever spec of dirt in the world. Impossible to clean off. Feels tacky to the touch now.

Make test pieces with whatever you plan to use