Since we are using cubic foot for comparisons, here are some to look at...
steel/iron runs in the 490's lbs.
Aluminum is around 170's lbs.
Rubber is around 94 lbs.
leather is around 60 lbs.
lead is around 708 lbs.
copper is around 560 lbs.
Delrin aka acetal is 87 lbs.
Nylon is around the 70's lbs.
PVC is 87 lbs.
Lexan aka Polycarbonates is 75 lbs
Teflon aka PTFE is 135 lbs.
For reference goto: https://www.engineeringtoolbox.com/density-solids-d_1265.html
okay...
chuckling @switchablesusie, "Delrin as a cane is damn near deadly feeling at 88 pounds per cubic foot. A 3/8" diameter Delrin cane will make you hope death happens soon." Yes Delrin is rather stiff, dense and with round rod of 3/8" does deliver a deep penetrating thud unlike rattan which is very light.
@chester89, Thank you!
Now I know we said we wanted more thud, my question would be is what has been used so far for reference in paddles used so far on this person. Is bruising okay with that person or not, just what type of thud as a real heavy flogger can be thuddy, rubber products can be rather thuddy and do deep penetration and possible bruising. You see above I have listed some materials that paddles could be made from or in combination. Aluminum could be a starting point for something heavy...
Usually a smaller paddle with more weight will be thuddy, lighter tends to be more stingy...