glass dividers are not needed. Acrylic sheet, 3/8" is available at most hardware stores, and is easer to cut if you have any kind of saw. Aquarium sylicone or the Sylicone#1 will work and is reef safe. It will stick to both glass and acrylic. I have a 20g glass tank with acrylic dividers syliconed in. Just make sure to get the thicker acrylic. dont get the 1/4. 3/8" or even 1/2 is better, especially for a 18" tall tank. YOu Wont want your Baffles all the way to the top, but they still should be 12" up from bottom on the first section for skimmer, heater, intake from DT. Middle section is your refugium, sand+Micro Algae. 13" Baffel bottom up, with 1" Teeth, spaced every 1/2 inch, 3/4inch gap, 14 inch baffel top down, 1 more 13" Baffel with 1" Teeth, and leave yourself 5-6 inches for return pump and plumbing, and you are Done. If the tank is more tall than wide, u can put a HOB Skimmer(i have this) in the sump and leave the heater/intake section narrower. Just get a Sponge cover for the intake on the skimmer(many come with it), and put the skimmer in the middle section with your Algae.
Acrylic sheet is much easier to handle, and put teeth in than glass.
Im good at custome designed refugiums. If you need detailed plans, let me know.