gday mangonui
im thinking the beer cooler your talking about may be equivalent to what we call a temprite.the beer coils inside are made from stainless steel.i have yet to see a stainless system for holding lobster.when dealing with lobster etc titanium is a good option if not the only viable one.anything less is liable to kill them .your cheapest option is maybe to bend up a few lengths and dump it in your water circuit.probably pay to research correct pvc piping and tank materials also.
good luck