Perspex is one of the common material to represent human tissue, different thickness (layers) may represent different part of human body. Aluminum layer used to represent bone.

Please refer to the method of this study: "Visualisation of uric acid renal calculi (UARC) using computed radiography (CR)" doi:10.1016/j.radi.2009.11.001

Reference: Gray, Winkler, Stears & Frank, Quality Control in Diagnoatic Imaging.

PMMA (acrylic) is a preferable material for phantom, since the price is OK and it can be made become any geometrical shape you prefer!!

Lung-Kwang Pan; do you have any publications or methodology using PMMA to design phantoms?

check my published papers, there are several ones to explore the various purpose, external dose, internal dose or biokinetic model as well

As a first step you have to calculate the absorption, beam hardening and scattering properties with the ICRP model (Adam and Eve and a child). As a second step you can calculate with different materials and thicknesses how close they match the ICRP model. If phantoms should be designed to check the image quality the beam characteristic and scattering has a dominat role; the absorption is not of interest, means the entrance dose.of the phantom. Only in that manner the dose at receptor represents the clinical situation. In general different combinations of PMMA, Al, CF2 and air gaps  can be used to simulate the interaction of human tissue and X rays.

You can calculate effective atomic number (Zeff) for tissues with Phy-X/ZeXTRa. https://phy-x.net/ZeXTRa

Phy-X/ZeXTRa is very efficient, easy and accurate for your requirement. Use and enjoy.