How to pack speakers w/out original boxes?

For packing material use swimming pool noodles. They are these hollow closed cell foam tubes about 4 feet long. Kids use them to help stay afloat. They are cheap in off-season if you can find them. Just cut them with a knife to the size you need.

Once you have the boxes, use ridged Styrofoam, and plenty of it on all sides, not anything soft. Snuggle is the way to go for it their is any movement in the box itself damage will result.

You've had some good suggestions so far. I've used structural foam from a store like Home Depot or Lowes. Get the good stuff, not the white, brittle stuff. Once you find decent boxes, use at least 1.5-3 inches of structural foam on all sides, top and bottom. Get a couple of different thicknesses, if necessary, to add up to the width you need. Use a drywall T for marking and as a straight edge for cutting the foam with a utility knife. Make sure the wrapped speakers fit tightly in the foamed boxes so there is ZERO movement. This will eliminate breakdown of the foam and possible abrasions. Clean the speaker with a damp cloth, then dry, before wrapping the speaker with stretch plastic kitchen wrap. Stretch the plastic wrap tightly around all sides, top and bottom of the speakers. The plastic wrap will eliminate any abrasions.

I've shipped a few different speakers and audio components this way and the packaging I created was WAY better than most factory packaging I've seen.

Being fairly paranoid about shipping, when I sold an amplifier to someone in Thailand. I double boxed it with foam between the boxes but went beyond that and used construction adhesive to bond 1/8" plywood inside each of the four surfaces of the outer box. That worked well both to make the box more rigid and, theoretically, to prevent something from penetrating the cardboard and foam, like the toe of a delivery driver's boot, for instance.

I agree with the foam insulation from Home Depot recommendation. I used 2" thick (4' x 8' board) and wrapped the foam covered speakers with cardboard that came with my plasma TV. Lots and lots of 2" packing tape.

I sent my speakers cross country and the receiver raved about the job and speakers were fine.