Any lower will do, if going the Cleo route then download the form 1 from wibsite titleii.com, download form 5330.20 from there also, get your self a set of ATF marked print cards, from ATF website or a dealer, fill out forms attatch photos duplicate form 4s duplicate prints, one 5330.20. Get the CLEO signature send all forms to ATF with a check for $200 and wait.
Name/city/state or accepted abbreviations must be on the receiver or barrel in plain sight.
If you do not want additional markings you can buy a factory SBR or ship the lower you have back to the OEM and they can register and form 4 direct to you across state lines no local dealer needed, if they will.