Hardware-wise, you can get HiSoft's Surf Squirrel PCMCIA-to-SCSI adapter and attach an external SCSI CD-ROM drive. For this, you don't need to open up the A1200, but then you can't use the PCMCIA for a network card at the same time. Note the following caveat: The surf squirrel does not support auto-booting from a cold boot (power cycle) however drivers can be made resident by booting from another HD, resetting and then the HD attached to the surf squirrel can be booted.
Or you can get a trapdoor SCSI controller (or accelerator with a SCSI interface) and connect the pigtail to the unused port on the back of the Amiga. Into that you would plug the external SCSI CD-ROM drive. Some or all such controllers support auto-booting from a cold boot.
For the above two solutions, you can get a SCSI-to-IDE adapter and use an IDE CD-ROM drive in place of the SCSI one.
Or you can get a short 3.5" to 2.5" IDE adapter to connect an IDE CD-ROM drive to the IDE connector on the A1200 motherboard. But the IDE controller in the Amiga 1200 is unbuffered and can be picky about what you connect to it and how long the cable can be. Also, you have to figure out how to route the cable out of the Amiga 1200 case.
Or you can add a buffered IDE controller like the Elbox FastATA or 4xEIDE'99 and plug in probably any IDE CD-ROM drive, with longer cables if desired.
Also there's this slimline CD-ROM drive with 2.5" cable designed for the Amiga 1200, but I don't know how well it works.
I believe the IDE-only solutions above have the same auto-boot caveat as the Surf Squirrel.
Then there's the Rapid Road USB controller for the Amiga 1200. Maybe it will support a USB CD-ROM drive, but it has no autoboot support. For that, you would need a Deneb USB interface plugged into something like a Mikronik Zorro-2 board, but I don't know if it will boot a CD-ROM drive directly or if it needs a custom Kickstart with CDFS support.
Or you can convert an Amiga CD32 to an Amiga 1200 with the DCE SX32 or SX32 Pro, but the CD32 and especially the SX32 are hard to find. Similarly, you can use a Eureka Communicator to attach the CD32 as a CD-ROM drive to the serial port of an Amiga 1200, but the A1200's built-in serial port is only reliable up to 76,800 baud (about 7.7 KiB/s) whereas a 1x CD-ROM drive is 150 KiB/s.
Or you can copy the contents of the CD-ROM to a CompactFlash drive and use an IDE adapter to attach it to the IDE port inside the A1200, or use a PCMCIA adapter and EasyADF software. Or you can use WINUAE to install the software from CD-ROM onto your hard drive image. (This is how I installed ClassicWB.)
The best options for booting from CD are probably the trapdoor controller or the converted CD32. Otherwise you may need to first boot from an emergency boot disk, do the warm boot workaround described above, or build your own Kickstart ROM as described above.
As you can see, there isn't any one perfect solution, but I've listed all the hardware options I can think of.