If u don't wanna make it they sell Tank Dividers Tank Divider from Amazon$15 But 5 of those will cost u $75.
Buying enough acrylic from home depot plus the silicone might cost u around the same.
Silicone $12.19 from amazon
One tube should be enough but if you haven't worked with silicone before its kinda annoying.
I would think you cut small pieces of acrylic to silicone permanently in that would Hold the main piece in. Since it wouldn't be holding pressure that should be strong enough to hold but I'm not certain on it.