FrameTek's EconoSpace is hollow to make it lighter and stiffer than if it were made solid. It has 250% thicker adhesive than any other spacer. It is also easier to cut if you can read and follow the instructions. EconoSpace also has a nice satin finish on one side so you have a choice of glossy or satin showing towards the art. EconoSpace comes in 4 sizes: 1/16, 1/8, 1/4, and 3/8 inch airspace. Most sizes available in clear, black, and white. They all come a full 60" long, not 4 ft 8 inches.
For those using adhesive-backed spacers (such as EconoSpace) and who may be concerned with eventual slippage, I recommend installing them in "post and lintel" fashion. That is, install the top and bottom spacers first, and then the sides to fit snugly with the top and bottom. That way, the top spacer is resting on the two side spacers. If you do the sides first, the top has no additional support at the ends. If you "windmill" them, it has no additional support at one end.
Post and lintel is what we do. I've heard to do pinwheel, but it seems like one end of the top wouldn't be supported.