A layer of masking/painter’s tape on the bed itself has been shown to reduce flashback in many circumstances. It’s really important that the entire piece be flat during operations. I haven’t tried leather yet, but the air assist can (and has!) blown into paper that’s been edge-taped and blow it up like a balloon, causing all kind of havoc.
Again, I haven’t tried this with leather, but something along my DIY sticky mat might be helpful to you: