BrianChia
10+ year member
CarAudio.com Elite
Plexiglas should be fine as long as it is very thick and sturdy so it has minimal flex.
Some notes of the box: I would try using some poly-fill or acousta-stuff inside. The downfiring box is about 2.63 cubes net before driver displacement. Stuffing will make the subs "think" the box is bigger, which will lower the Q and they should hit a little lower.
Also the box is quite long so you may want some minor horizontal and vertical cross bracing as you feel appropriate to keep the sides and top rigid.
Some notes of the box: I would try using some poly-fill or acousta-stuff inside. The downfiring box is about 2.63 cubes net before driver displacement. Stuffing will make the subs "think" the box is bigger, which will lower the Q and they should hit a little lower.
Also the box is quite long so you may want some minor horizontal and vertical cross bracing as you feel appropriate to keep the sides and top rigid.