If you make a "plate" chuck by mounting a piece of flat to a faceplate, you should run some adhesive between them to seal; hot-melt is perfect for this.
A note of caution on vacuum chucking is worth making at this point. Many don't understand or forget that with vacuum fixing, your piece is being held on the chuck by atmosphereic pressure, which is, say 15 lbs per square inch. When you hold the piece to the seal and start the pump, the amount of physical pressure PUSHING THE BOWL onto the chuck is determined by the surface area inside the seal's contact ring as a function of the amount of vacuum created. So, if you could pull 30" of vacuum and your seal covered 1 sq. in of area on the bowl, the workpiece would be held as if you placed a 15 lb weight on it. Now, say you make your chuck 6" in diameter and your seal is at the rim, that it all fits inside your bowl. You turn on your pump and pull 30" of vacuum (full vacuum) you can calculate the weight holding your bowl as 3.14 (Pi) x 9 (radius squared) x 15 (air pressure) and you'll find you have 424 pounds pushing on your wood. Best consider your bowl, how thin and strong it is and whether it can survive a 400lb weight pushing against it.
Keep in mind that the example is for a "hard" vacuum at sea level. Any vacuum pump in a woodturner's price range will, at most, pull a bit over 28" and usually a bunch less than that due to imperfect seals in the system. The point, however, is with vacuum chucking you walk a line between not enough pressure to hold the bowl and so much that the piece implodes, usually when you just finished sanding the bottom, you here a "pop", and discover that you have a new funnel or a design opportunity. Thus, pick a chuck whose area under seal will provide enough area to develop the weight needed to hold the piece, but also have a bleeder valve in your vac line within reach so that you can reduce the vacuum (and so the pressure holding the bowl on) if you start to hear your piece creaking and making sounds.
Note further that trying to get a good seal with the rim of a bowl is difficult, if not impossible; it's rare that a bowl runs so deadnuts flat at the rim that it will make such a seal as is needed for vac holding. I see most turners going to drum chucks in a variety of diameters (I use 4, 6, and 8" versions). These are easy to make from Schedule 40 PVC pipe fittings, a 3" faceplate, and some MDF. They also have the advantage of making the vacuum seal on the inside bottom of a bowl where it's likely to be the best place to get a good seal anyway.
Hope this helps
Mark