I have a buddy who works with aluminum so fabrication is a possibility. I have a 1542 riveted alumacraft boat and will only be running a longtail motor.

1) Are aluminum Flotation Pods hollow in the inside?
2) Can you have a 1 piece rectangular prism floatation pod as opposed to the usual 2 piece? Say 40in*8in*10in as an example
3) Will the 1 long flotation pod ruin or better performance?
4) Will the 2 piece flotation pods give better performance?

flotation pods are typically hollow on the inside. I guess you could foam fill them but it is not needed. I am not sure I follow you on a 1 piece vs. 2 piece pod. Go to beavertail site and take a look at their pods to get an idea. The only real limitation I see with any pods is that you make sure the motor clears them when turning. Props and aluminum don't get along well. You can probably buy a set of pre-fabbed pods and have your buddy install them per provided instructions for much cheaper than you can buy material and trial and error your way into a homemade design.