If you have a HOB filter just plug the CO2 output into the filter intake that's what I do in my 20 gallon works fine over time it's said that CO2 will harden the impeller but I haven't had any problems so far. As far as cheaper DIY vs buying a diffuser that depends on the type of setup and to what degree you want to diffuse if it's a big tank glass diffuse that bad boy smaller tanks can probably get away with an airstone if the tank is in the mid to lower range of light conditions. There are some variables you can consider hope this helps.
Are you using pressurized or DIY CO2? If it's pressurized, I'd just go ahead and buy a good diffuser and be done with it. DIY has trouble developing the pressure required to get some ceramic and glass diffusers working. If you are able to make or buy a reactor, that is a better option still, whether DIY or pressurized.
A lot of people inject CO2 into the inlets of their filters or powerheads. Depending on the setup this can be annoyingly loud or more or less silent.