I bend 1/4" HDPE with a heat gun all the time.
3/8 " just needs more heat or patience ?? Test on scrap.
EDIT/ OOPS. Went into the shop to check and the HDPE I've bent is 1/8". My mill enclosure among other things.
The problem with this stuff as it gets thicker is the outside can be heated to melting without warming the core.
More of my rubbish advice unfortunately.