just measured mine for ya, it's 6 1/2"wide x 9" long, and 6 1/2" deep, i considered making one too, before i got a great deal on this one, just like you said, bend, leave corner tabs, and rivet, then seal with silicon, and i was gonna use 2" pvc electrical conduit for the tube, being an electrician for years, i've bent many a pieces of that, just take your time with a torch, keeping it moving so i dont burn it in one spot, till it gets all rubberry, have a template to hold it on the floor to, till you get the right angles to make it around the shock, and then hit it with a wet rag to cool it in the shape you need, then epoxy or quick steel it into the box, it needs to stick in about an inch, so the pod style filter can clamp to it, the uni i just got says for 2 1/4 - 2 1/2" tubes
hope that helps ya, post up some pics if ya get 'er done