Basically there isnt any way to control DIY CO2 without possibly having a mess on your hands. What most prople do is run an airstone at night to gas off the CO2.
For an pressurized CO2 system you are looking at about $200. If you are going to upgrade buy qualtiy stiff the first time then you wont have to upgrade in 6 months to a year.
You can make a DIY Inline reactor. If you search for it with the search option you will find a couple different designs.