What better way to transfer heat than to use a glass plate. The glass plate transfers heat very well. The glass is also inexpensive - glass plate is less expensive than Plexiglas. I simply used some picture frame glass that I already had laying around the house. I cut two plates 18" x 3 1/2" with the idea of sealing the LEDs between the two plates. The open gap around the edge of the glass is then sealed with a bead of silicon sealant. Once sealed, the glass seems very solid - the two plates glued together makes them much stronger.
During assembly, the LED carrier boards are super-glued right onto the glass. I used 24 LEDs in total. Of the 24 LEDs, 5 are warm white and 19 are blue. This gives me 125 lumens of warm white and 114 lumens of blue.