If you have access to a table saw, cut two sections of PVC pipe and make a cradle of them like this, but rotated 90 degrees )( .
A drilled out tornado tube coupler will hold the bottles quite well.
You might look at Home Depot or a plumbing shop for O-rings that would fit over the mouth of the bottle, but atop the ridge below the threads. That would seal to the tornado tube coupler better.
My complaint would be how difficult it will be to put media inside the bottles.