A lot of factors (and opinions) when it comes to free choice/protein feeding. I have friends who use the gravity feeder tube from Academy with great success. Personally I prefer the Bossbuck Gravity Head tied to PVC and a 55 Gallon Barrel.
You will find people have had success with one brand in their area while others have better success with a different brand. A very common theme is that when you first start protein, you may have better success if you combine it with corn initially and over time lessen the corn percentage until they are on 100% protein.
Some factors to consider when deciding: (1) Will hogs or livestock have access to your protein feeder (2) What brands of protein do you have local access to and (3) How far is your drive to check and/or refill feeders. With unknown consumption, this will be a key factor.
Some common brands in no particular order are Lyssy & Eckel, Purina (Antler Max), Record Rack, Mumme's, and Red Chain. I'm sure others will follow up with additonal brands/suggestions I have missed but wanted to throw out some starting thoughts.
The blood trail that really matters leads to the cross!