Good alternative to HDPE for FDM
According to the chart on this page HDPE has a flexural modulus of between 0.75 and 1.575 Gpa; meanwhile PETG has a flexural modulus of 2.20 Gpa, meaning PETG is the stiffer material.

HDPE, by definition, has a density greater than 0.941g/cc.  PETG has a density of about 1.25g/cc.

So while PETG may be denser (depending on the HDPE used), it is more rigid, while not being too rigid.

PETG is quite printable using FDM.  If you were printing at a higher temperature and high % infill, I could see it holding up well enough.  I've printed heavy duty parts with it using a 0.8mm nozzle.