We get asked if a spacer would work to fill gaps that are only 2 or 3 inches wide. Theoretically, you can, but we don't recommend substituting in spacers unless your installer is confident they can cut the smaller 5" width exactly to get the two pieces (visible and backing) you need.
Our fillers come 10" long in part to give you some wiggle room. Cutting a perfectly straight line parallel to the edge of the filler is not as easy as it sounds, especially if you're making a cut 95" long.
Before substituting a spacer for a filler, talk with your installer and see what they'd prefer. Keep in mind that if a spacer is cut incorrectly and you need to reorder it, you'll need to place a new order, delaying your project.