First one is ok but you have to replace wooden airstone every ~2-3 months. Second and third solution is not effective at all. The last method is ok but some of the gas bubbles still going trough the filter and being wasted, bubbles which was not dissolved inside the filter flow out trough outlet pipe and gone, anyway at least is not elegant way.
I decided to make my own mixer from water filter housing.
Basically it's not my idea, and you can find plenty of similar solutions on Internet, but I'll try to describe how mine is made.
Step 1: General idea and overview
The mixer is made from in-line water filter housing. inside is handful of bioballs to make CO2 diffusion more effective.
The trickiest part of the job I've found, was to find proper connectors to filter to connect ordinary filter hose to mixer and connect CO2 on the inlet.
After nearly an hour of digging in B&Q plumbing department I've found plastic connectors for standard 15mm water pipe. When I removed nut from one end, I found the same 3/4 inch thread. Luckily, even better in terms of fitting/sealing, after the thread was a collar, so I was able to use rubber washer instead of sealing by normal Teflon tape or silicone sealant (you can see everything on the photos).