Brilhart mpcs are great and there are plenty of them for sale all the time. The ideal thing is to get older Ebolin ones that have the largest facing number you can find and without the star. The star on Brilys means short facing and what you want is the regular or long facing. Regardless of the number, the measured opening will most likely be different since their control over that was erratic and the marked size and actual size rarely matched. I owned a bunch of ordinary Ebolins years back and enjoyed playing them, but then I found a fantastic sounding Ebolin Streamline and that became my main tenor piece until I began to play in loud jams and needed something with a bigger opening and high baffle. Streamlines are one of the best models of Brilhart but they're not as cheap as Ebolins. If you find one at a low price, snatch it up--you won't be disappointed. I still play it at home because it is just so good especially since I had it opened and cleaned up by Theo Wanne a number of years ago and it got even better. It's the mouthpiece on the right in this photo below.
You'll notice that it has a hose clamp on it, which is to keep the obvious crack that you can see in the shank from spreading farther. Cracks in the shanks of Brilhart Ebolin and Tonalin mpcs are commonplace because the plastic can crack easily if it is pushed onto a new neck cork that is too tight. It's really just a cosmetic problem and doesn't affect the playing, although it might have an effect on the selling price. I put a bit of superglue in that crack before banding it, so It's not going to get worse.
Next to it I put my old 1950's Rico Reloplex plastic mpc made by M.C. Gregory that came with my 1945 Martin tenor when I got it years ago. I assume that it was played by the sax's owner, a man who had died. His trombone playing buddy was who sold the sax on Ebay for the guy's widow. He didn't know any more about saxes than she did but knew how to sell online because the mouthpiece was a surprise. When the sax came I immediately played it with the Reloplex and it sounded really good as I remember, but when I had the sax set up properly and the old neck cork changed to fit my mouthpieces, the Reloplex no longer fit because it has a smaller diameter bore. Later on I also had to band it with a hose clamp because it developed very light cracks in the shank that go around the circumference rather than down the bore from trying to force it on the neck cork.
The reason I am showing it with the Brily is so you can see the close similarity in the design of the two pieces which would suggest that this Reloplex plays a lot like a Brilhart. Notice that they both have a slight bit of roll over baffle at the tip, straight sidewalls, an Arched throat that is wider at the bottom with a bit of hollow in the floor of the large open round chamber. About the only differences I can find in these mpcs is that the Brilhart has a smaller tip opening at .081, even after being opened by Theo, than the Reloplex which was and is .090 from the getgo. It's marked a 5P and has never been worked on. So while they may have a similar sound quality the Reloplex will naturally be louder.
Supposedly Elmer Beechler oversaw the making of this mpc for Rico--according to Theo's Mouthpiece Museum page on Gregory Mpcs--but that doesn't necessarily mean he or anyone at Rico couldn't have just copied the already successful design of the plastic Brilhart mpc already going strong. I would even wager that that they did since since their own mpcs were HR and not plastic which is why they had to get Rico to make them for them. Copying the Brilhart design would have saved them a good deal of time and cost.
So the reason I've mentioned it at all is that while Vintage Brilharts are between $250 and $400, Reloplexes go for less than $150 and often they don't even go at all. They used to be priced around $60 but that seems to have doubled lately. They aren't hot items--I tried to sell mine years ago for something like $75 and nobody cared, so I kept it. Thus I imagine you could find an alto model in good condition of the right tip opening for you and make an offer the seller wouldn't refuse. Even if it cost you a bit over $100, it would be little to spend for something that may be just what you're looking for. The only caveat I want to give is that I have never played an alto one and so don't know how they play.