I use a nutrafin co2 fermentor i bought on ebay for approx £10 made by hagen.
it is just a canister with a tube coming out of it basically, easily made at home i would imagine, with a bottle, some Co2 tubing and some form of sealant.
It also came with a bubbe ladder (diffuser) This would be much harder to make, There are several of these on the market, varying in price and complexity i would reccomend buying one of the cheaper ones in the spirit of the DIY method. My bubble ladder seems to work quite well, big bubbles at the bottom and tiny ones that finally escape up to the surface. there is some loss, but much less than if the gas just bubbled straight up to the top.
My shrimp are fine and breeding and have had the co2 since before i got them, but my water is quite hard, so buffers well against the acidifying effects of co2. ins softer water areas this may be more of a problem.
Diy is not as good as pressureized Co2 but is effective and cheap.
My plants grow well, just a few algae problems... but seems pretty well sorted by my easy carbo (excel equivalent).
I'm sure others will have lots to say, and loads of info can be found at
Some on that site are very elaborate, but some are easy and increase co2 absorbtion