I like Brian Brays design for the slatted rack, using dowels. Would it be ok to use aluminum tubing for the dowels?
PVC will work okay as long as the outer diameter is no more than 1" but metal of any kind draws to much cold, it would be like asking your bees to enter and exit through a refrigerator on cloudy or cool days.
I like slatted racks I use them on all of my production hives. The are a necessity on bottomless hives. Even the strongest hives do little to no bearding when a slatted rack is used.
When I recently changed from bottomless hives back to SBB due to a change in hive stands I still kept the slatted racks in place.
I currently equip each hive with:
Screened bottom board
Slatted rack (slats lengthwise not crosswise)
3-4 medium 8 frame brood boxes
Reduced upper entrance directly above lower entrance (inverted solid BB work well for this)