There are rib racks that do a great job with baby back/loin back ribs. In a pinch they may even work with St. Louis style spare ribs but fail miserably when they attempt to hold up a full rack of pork spare ribs. While digging around in the garage looking for a solution I found a wire basket I bought to grill vegetables in. Inverted this did nicely to solve my problem. I laid down one rack of pork spare ribs and one rack of beef ribs then the basket and draped a second rack of pork spare ribs over the basket. This allowed plenty of smoke travel above and below all three racks. It was fairly easy to rotate the meat as well since I just pulled out the basket with the ribs on it,  rotated the bottom ribs and then put the basket in 180's opposite how it came out. The only negative to this setup was the door thermometer which was too long keeping the door from closeing. I popped it out and things went fine after that but I didn't know how hot the smoke chamber was.