How to pack speakers w/out original boxes?

We ship aerospace parts. No matter what box you use buy four of them. Cut two of the boxes up and line the assembled boxes with the cut up cardboard pieces. You will in essence be building a double walled box. Much better and stronger. Double tape all the sides/corners of the assembled boxes on the outside.
Using the methods noted above, you should be in good shape.
Good luck, John

"How to pack speakers without the original boxes?"
DON'T!

Very good advice here and just wanted to add....I use the heavy duty extra large trash bags to wrap speakers in to keep them clean from the styrofoam debris.

Use the 2 and/or 4 inch round foam noodles especially in each corner. They are usually sold in 4 foot lengths, and can be cut to size across or in half lenghtwise. Use them and double box even if you have the original boxes.

Whatever you do, don't let the UPS store package anything of value for you. They completely destroyed a pair of PBN speakers that they packed for me a few years ago. I usually don't have problems with shipping claims, but that time it was a real bear to get them to pay up.

They used a single outer box (no inner) with popcorn packing material for an 90 lb. speaker. That's it. No end caps, no hard or soft foam. Just popcorn.

Shakey