Forgive me if this sounds ignorant but would it be just as simple as building an air tight container and hooking up a shop vac? Or does that not have enough Suction? Any DIYers out there that have made one?
While shop vacs do provide suction, they are not designed to produce or maintain a vacuum. You would need to get a vacuum pump to handle that.
Here is an example of an extremely inexpensive and simple evacuator built from harbor freight parts.
On his setup there are a couple things I would change. I would add an additional valve to the top for letting air back in when done, or temporarily reducing the vacuum pressure in the event that your material starts to bubble up to the top of your container.
I would also do a different gasket design. One great method is to use a thicker plastic sheet for the lid and CNC out a circular channel that matches the diameter of your pot. You can then pour rubber into that channel and have a gasket built right into the lid.
I have made an excellent vacuum pump from a recycles fridge compressor.
There are several video's on it and it's very easy. For the actual pot - look in a second hand store for an old aluminum stock pot, the real heavy duty ones.
In this pic you can see my unit which also doubles as a compressor with an old fire extinguisher bottle as a tank.
You can generally get the compressors from a junk yard. hose fittings/tubing from a hardware store. A pressure switch from a electrical store..
10mm plastic for the top can be got from a recycle store. It is faster and easier to put the fittings through the side of the pot rather then drilling the plastic. Less strain on it as well.
My compressor can pump up to 70psi, and take the vacuums right down for degassing. And it's pretty fast.
Oh - its silent as well
Re the changes - personally I'd avoided any cutting of the disk and use ports through the side of the pot. The issue with the top is that ANY hole adds a strain pint and could lead to fatigue cracks. Seen my original pot lit go like that.
Also sealing the pot is easy as one you have drilled the holes for the inlet and outlet (simple valve) and installed ports - your can use 2 part epoxy to seal by slightly charging the pot with a vacuum and then applying the epoxy around the joins. After it has set you should have a great seal.
edited February 2016
I used a similar method to what Chris described for the seal. I have a 15mm acrylic sheet for the lid and I used a router to cut a ~5mm deep channel using a brass dowel that I machined to slip into the bushing that the manifold is attached to, to centre the radius of the cut.
I then poured PT Flex 20 polyurethane rubber into the channel, to about 1.5mm from the top so that it locates easily on my cooking pot. It seals beautifully.
All up cost was only around £70 ($100 USD). £42 for the vacuum pump, £13 for two radiator fittings (1/2" BSP to 15mm pipe fitting) and two 15mm ball valves, a few quid for the hose and £6 for the vacuum gauge. I machined a custom manifold out of 2" hex brass (I know not everyone has that luxury of a lathe, but it can be done with a decent drill press, a few taps and a die - I used a 1/2" BSP tap, 1/4" BSP tap and 1/4" BSP die).
The pump is 3 cu.ft/m, the pot is around 0.25cu.ft and it gets down to almost complete vacuum in around 70 seconds. Might even be quick enough for alginate. The ultimate pressure of the pump is allegedly 5 pascals. It's enough to boil water at room temperature
I have a ball valve on the input and output of the manifold, so once the pump has attained maximum vacuum, I can close the input valve and turn off the pump, then when I'm ready to open the chamber, I can slowly vent air back into the pot using the valve on the output.
Awesome job, I'm jealous. I need one for my shop!
With your skill set, this would be a piece of cake for you lol.