Alright, I think I have come up with something.
The skimmer is based off of a RK2-150 protein skimmer - for reference, a RK2-25 or a RK2-50 could easily run on a 3,000+ gallon tank housing up to 4 whitetip reef sharks (this was from a post by someone in the public aquarium industry). So a 150 is a massive skimmer - that retails for about $9,500. To build this thing, it would start with a 6.5 foot high main body with an end cap on the end, made from 36" diameter PVC. Harrison Plastics is an online store that sells PVC up to 5 feet in diameter which would be more than enough to build a truly massive skimmer, like the ones used on massive public aquariums (the RK2-2000 is the largest one and it retails for about $50,000).
After the body is built, the inlet and outlet would have to be added. The inlet would be a simple PVC pipe with a ball valve or two on there for flow control. It would be connected to a 1hp pump with a venturi valve before this - Lifereef is the only company I have found that sells these on their own. The outlet would be a simple PVC pipe with a ball valve or two on it for flow, again. No need to make anything more complicated than it has to be.
Next would come the collection cup - to start, some type of acrylic cone would need to be built, and the top piece cut off for the bottleneck part of the skimmer that the bubbles spill out of (not sure of a technical name). This would be sandwiched between two polymer plates and attached to the top of the 36" diameter PVC. Note that the diameter of the collection cup is 20" and it is to be 24" in height - this is proportional to the dimensions of the RK2 skimmer of equal size.
The collection cup would be just what it sounds like - a collection cup, again nothing fancy. Made from clear acrylic tubing, 20" in diameter, 24" high. Removable. That's about all I've got.
I think I've covered pretty much everything of this except for price. This doesn't have all of the bells and whistles that the RK2's have but it's a big skimmer that does its job. Costs really break down to:
6.5' length of 36" PVC - well, a 10 foot length of 24" PVC costs about $1,000 new, so this would be the most expensive part - however you could probably make a few good connections and get a scrap piece of 36" for $500 no problem
Custom acrylic collection cup - assuming it's DIY, the acrylic would cost about $120 plus $10 for a can of some adhesive to hold it together
Your end cap for the bottom is probably going to cost about $120 (very rough estimate, I honestly don't have too much of a clue on this but that's my best guess)
PVC pipe and fittings for the inlet/outlet(s) should be given a $350 budget
Figure we will run six venturi valves on this (I am talking to Jess at LifeReef about the configuration) and that will cost about $500 for them all
The acrylic needed to build the bottleneck piece between the body and the collection cup will probably cost $60
Various nuts and bolts and plates and such to connect the acrylic bottleneck, the main body, and the collection cup will get a $50 budget
The pump is about $450
So for all of that, you're going to pay (as an estimate) $1,800 (that gives you an extra $90 in case I estimated anything wrong). That's 20% of the cost of the similarly-sized RK2, so it's not that expensive in comparison. Let me know if I have any flaws in there
