For a neutral background I use either a dark grey felt or a light grey felt cloth, whatever works best w/ the particular mib & a suitable rubber "O" ring to hold it in place (if needed) There are exceptions where I'll use black or white foam but very rarely, sometimes it takes a number (alot) of retakes w/ different colored backgrounds but eventually I hopefully get there. Ah the wonders of modern digital photography, in other words it's a good thing I'm not relying on film or I'd be picture poor ....