HDPE sign installation

Stud mount it.
Predrill holes 1 size larger than your studs. Preferably with a drill press so you don't go all the way through accidentally.
Join studs using epoxy.

"High-density polyethylene (HDPE) is used for milk cartons, fuel tanks and pipes. Previously, HDPE could only be welded together by melting the surface of the HDPE. However, recent innovations have created epoxy-based glues that allow HDPE to be attached to other polymers such as PVC, fiberglass and steel. Different brands of HDPE glue, like Pro-poly and TAP Poly-Weld, will have different manufacturer's instructions. However, what they do have is common is a need to prepare the surface of the HDPE to accept the glue."

 

tape won't work on HDPE. there's specialty adhesives out there that work well...enough at laminating a sheet to another material but for most sign installation purposes there's no amazing chemical option out there that really "bites" into the material on its own.
If you attempt to use any adhesive, however, +1 to surface prepbeing HUUUUGE: scuffing/rouging & heat-treating/torching.

we usually stud-mount thicker panels, when visible hardware is an eyesore, and screw/thru-bolt thinner panels.
most recently, per a provided spec., we stud-welded a sheet of 1/8" aluminum and bonded the 3/16" sheet of HDPE to it with 3M 8010 2Part Structural Plastic Epoxy. Uncertain how well it would adhere to anything but a perfectly-flat surface, but it bonded a 3ftx5ft panel of HDPEtoAluminum and currently holds it vertically on an exterior directory.

 

Sand, clean with xylol flame treat and West System Six10, I have also used 3M 5200 but that is just to seal. You have to put your woodworking skills (dado, mortice & tendons, countersink, etc.) to work when mounting and fabrication. Great product for outside for holding up to environment.