Maybe buy some tough plastic drainage pipe of an appropriate diameter/length, and use lots of bubblewrap to secure in place.

first and foremost you must remove the bases and pack them separately, just like B&O did to ship them from the factory. As far as packing materials, I prefer solid sheet styrofoam insulation which is available at Home Depot or equivalent in 4x8 sheets - 1" thick is sufficient in most cases. It's easy to work with and if you plan out the dimensions of your pieces before you buy it, you can cut it down to fit in your car when you're loading it in or on at the store. A utility knife and straight edge are all you need to cut it.

Measure the speakers without the bases to get the length, width and depth. Add 2.25 inches to the width and length and that will be the bottom and top pieces - you'll need 4 of those. Now add 1/4 inch to the depth and then the outer sides (4) will be the original length x depth(+1/4") - you need 4 of these.

Wrap the speakers in small bubble wrap making sure that no tape ever touches the speakers. I would cut two pieces that are the length of the speakers + depth for each speaker. Tape the two sheets together, lay this out and lay the speaker onto this sheet, grill side up.  Bring the sides up and lay one over the other and tape up. Finish up the top/bottom.

Set one bottom sheet of styrofoam down and tape the two side pieces to it to make a U shaped enclosure. Set the speaker into this and it should fit snugly now. Check the gap between the top edges of your side panels and the top of the bubble wrap around the speakers. If it's close enough, tape the top onto the package. Finish up by fitting ends caps, then tape the finished package as necessary. I use shrink wrap from U-Haul that comes on a small hand held roller and it very easy to use.

This will approximate the original packaging without any cardboard of course. While it may sound complicated, it really doesn't take very long at all if you plan it out on paper first and make sure you measure twice and cut once. The styrofoam sheet(s) will cost $20 ea.  The most difficult part of the operation is getting the styrofoam sheet(s) home from the store. I have a Volvo wagon so I can fit them in pretty easily.

Good luck!

Thanks everyone for the input - will look at a few options noted and see what would work.