1/8" HDPE is really thin but I was able to countersink it to my rail barely.  On my box
I used 1/4" and it's so much more solid.  But 1/8" is possible, just pretty sketch.  Also 8' is short dude.  I'd go with atleast 10' or
12'....or make 2 8' boxes like i did.s really think but I was able to countersink it to my rail.  On my box I used 1/4" and it's so much more solid.  But 1/8" is possible, just use coping i guess.  Also 8' is short dude.  I'd go with atleast 10' or 12'....or make 2 8' boxes like i did.