I'm using the access panels as heat exchangers, just like you've described. You can have them machined out of 5052 alloy and not get beat up too bad on the price.

We've taken bids from several machine shops across the country and Canada and gotten prices that range from about $50.00 to over $300.00. I'm planning to have the plates and heat tubes made in quantity to get the costs down. Unless you use the most expensive machine shops, even if you do it in single quantity, your cost shouldn't be more than about half the cost of the woofer, even if precision machined. If you do the machining yourself, the cost is even less.

Let us know how yours works out.