Vacuum table

Yes, I've built several but how difficult depends on many factors and the type of vacuum system you want and the type of parts you're going to want to hold. Also, the Vacuum pump you are going to use plays a big part.

To be honest, the bed is the easy part, it's just a Non-porus surface like HDPE with a grid pattern cut into it for the Vac seal which is divided into zones that can be isolated by turning off valves built into the plumbing.
The hard and very expensive part is selecting the right vacuum pump to suit your needs.

Vaccum work holding gets expensive very quickly, esp at this size, depending on the type of work you are wanting to hold.
If the parts are large and non-pourus with no or just a few holes in them then you can get away with a powerful vacuum cleaner. The more porous and smaller and the more holes you have in the part then the harder and more expensive it gets. For instance, I recently paid £3.5k just for a vacuum 7.5Kw pump for a 10x5 machine and that won't hold small parts without gasketing the parts individually.

Building an 8x4 Vac bed isn't difficult but doing it properly so it's useful and works well will be expensive.