I made spacers that serve two purposes. They are the lids to the boxes I made for the 650s as well as spacers. Works pretty well for setup. Put subs where you want them, lift off the lid and put it on top of the subs, and then put the 650s on top. They are 1.5" thick on the face and well braced on the sides with large holes that work as handles for lifting them. They also have slots cut in them for the interlocking skids on the TH118s.
That's very similar to what I have for my 3TX's, except they are not lids. I think they are 18x18x18 ply, screwed and glued, back and fronts, no sides, brace in the center between front/back, painted with truck bed paint. I use ratchet straps looped under bottom of sub dolly board and over the top of the mains. they are light.
But Brandon now you have given me an idea to build cases, and obviously make the covers a spacer - Thanks!