I thought about effective CO2 mixing for a long time. I've tested almost everything: diffuser made from wood, air stone, glass diffuser, I've tested CO2 injected on canister filter inlet, etc.
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

Mixer is placed behind the canister filter, so water is coming trough filter than water seize the CO2 bubbles during the flow trough Tee connector on mixer inlet.
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).

Step 2: Materials

Ok so all you need is:
- water filter housing 10 inch(in that example I've bought one from ebay (it was bargain price), but is standard RS part link and anyway it could be any of the enclosure available on the market.
- one straight trough connector 15mm pipe,
- one equal 15mm tee connector,
- 15mm pipe (unfortunately I had to buy 2m long pipe),
- hose clips
Also you'll need silicone sealant, saw (to cut pipes), small diameter pipe for CO2 inlet, a handful of bioballs (idea: I got my from filter, and replace by pieces of foam or replace completely medium to foam/) bioballs completely by foam/ceramic/**etc. (alternatively you don't have to use bioballs at all)

Step 3: Assembling #1

The rest of that job is assembling, and is pretty straight forward.
Cut 5 pieces of pipe 4 pieces approximately 5 cm long and one about 20cm. Fit two 5cm pieces into Tee connector and straight connector and tighten the caps. After that you'll have about 2 cm long pipe to connect filter hose. Remaining short piece of pipe you'll use to make CO2 inlet.
You have to find straight small diameter pipe about 7 cm long. (quite good source is an ordinary soap doser). Next you have to seal small pipe inside 15mm diameter 5 cm long piece of pipe. There's many ways to do it. You can use gluegun and then silicon sealant to be sure there's no leakage; or fill the 15mmm pipe by silicone then block on end of the small pipe and put it trough silicone (that is tricky but possible). Now its time to wait until the sealant dry out.

Step 4: Assembling #2

Now its time to put everything together.
Screw in both both connectors to the top section of water filter. Tee connector to In and straight connector to Out. Secure both by Teflon tape/silicone sealant/washer, doesn't matter which way you prefer unless its not leaking. I've bought for it rubber washers set (8 of them), and two of them exactly fit to connectors.Than you have to fit remaining 20cm long pipe in the lid. That will be the outlet from inside housing.
Put the bioballs (or not, its up to you) into bottom section of the housing and screw the lid together.
Now is time to make a decision where is the best place to cut filter outlet pipe in two.
Another small piece (approx. 8 cm) of outlet hose you'll need to make bubble counter (of course the hose must be transparent to see bubbles).
If the silicone dry out, you can fit CO2 inlet into 8 cm piece of hose and tight hose clip on it. From the other side of "bubble counter" should be Tee connector (the same way like small pieces of pipe inlet and outlet) and fasten the nut (see picture).
If you've decided where and cut the hose in two, its time to place hose clips on the hose, connect hose to mixer inlet/outlet pipes, and fasten up the clips. Hint: If its hard to pull on the hose onto a pipes. Hint: use hot water to soften the hose and and will be easier.
Connect CO2 hose to inlet and make sure everything is tight (you can secure CO2 hose by small cable tie or piece of wire).
If you're sure switch on the filter and will see how the new mixer is slowly filling up with the water. When the water will reach bottom of the inside outlet pipe will probably stops filling mixer, so at that point you have to press the pressure release button until mixer will fill up with water.
check the leakage and that's it.

Step 5: Heater

Ideas for a future.
That mixer is actually quite neat for placing heater. Main reason its just aesthetic. Basically You haven't got heater inside tank. Second reason is that the heat is spreading in tank more evenly (depends of inlet method of course). From the other hand, sometimes it might be dangerous. If for some reasons big amount of air will flow inside mixer and air level will reach the point when heater (or part of heater) will be exposed to air, the heater might crack. To prevent possible electric shock, worth connect heater to RCD switch. Anyway I strongly recommend unplug everything before you start digging inside the tank.
Hint: you can buy heater with water level sound alarm.

Step 6: Conclusion

It is probably one of the most effective way to dissolve CO2 into water. Also is one of the most accurate method of dissolving CO2. I'm using it for 2 weeks (~1 bubble/sec) no problems so far.
You can see happy fish an happy plants in the background.
(Sorry for language mistakes, there's probably plenty of)
Cheers