First-there are only 2 companies that make a kit that fits correctly. KIRK and Autopower.
Now to Why I use KIRK-the kit is fabricated to fit very tightly in a CRX whether you have the interior in it or not. It is available with a number of weld-in options. The price is very fair. The unit meets both SCCA and NASA rules for materials. Now, you must decide why and how you plan this to be in the car. If for the street only-then DON'T do it. For austocross and once in a while HPDE-get a 4 point with harness bar. If for the track the minimum will be 6-point and 8 is better still. If you are as serious as I am then get the NASCAR style door bars for track-only use. See where this is going.

Give KIRK a call. They are great to work with regardless of what you want the cage/bar assembly for. Once installed-please use the real SFI padding on it. This material is available from Summit Racing and is high density (minimum thickness is 1/2" but 1" is better).
Good luck