Here's the thing about Festool CT extractors - despite the size difference, the actual CFM suction difference is minimal between the CT12/25/MIDI and the CT 26/36/48 (130 vs 138). Meaning you can get the MIDI (my preference) and it will do everything most users need a CT to do - especially the variable suction to bring it to the lowest levels for sanding.
However, you can go with an adapter for your DeWalt - if funds are tight. Since I have the MIDI, I haven't the need for an adapter - meaning I don't know exactly which will work, but this one from TayTools looks like it will do the job (best to call them directly and ask them for clarification). But, like the others have noted, a typical shop vac will have suction that is prone to swirling. But, if you have the DeWalt Stealthsonic, it's rated at 100cfm - which may work in this application. My Ridgid 16ga has 146cfm - which is too forceful.
Taytools Xtract Electric Random Orbital Sander Adapters for Festool Dust Extractors and Shop Vac Extractors
taytools.com