I'm not sure of the grate size in your Masterbuilt XL. I tried to google for a source so I could read the specs...nada...anyway, a good fit for my Smoke Vault 24 with a grate size of 15" x 22.5 is a 12" x 18" aluminum cake pan. I can fit 3 slabs of spares with overlap & makeshift stacking, or 2 slabs St Louis, BBs or Loin Backs side-by-side in one pan.
Another possible choice for sizing is a full size disposable aluminum steam table pan, but these fill the grate space fairly tight in my SV24, so one of these pans is all you can use at one time, or baffling of heat occurs with anything above the lowest pan. I can still smoke other items below a pan, and do fine, but multiple pans are a serious problem. This baffling also reduces the smoker's convective efficiency, thereby increasing the overall cooking time. The full-size disposable pans are 4" deep, so you can have the ribs on racks standing on edge and fit 5 - 6 slabs per pan while tented. When smoking ribs or panning them while in racks, being vertical orientation, they seem to take longer to smoke and finish cooking. Also, if the slabs are touching each other, there will be cold spots and little to no bark formation or smoke reaction with the meat where they touch each other, and they can be undercooked in those areas, as well.
Vertical smokers can bring some challenges to overcome if you load them up with large items, as the baffling effect will always be there as a result of reduction in flow around the sides of the larger objects. This can be compensated for by periodically rotating the grate positions from top to bottom, etc, and running higher chamber temps. With the slightly smaller sized 12" x 18" pan, the baffling is still there, although not as prevelent as it is with the grate nearly completely covered with a full size steam table pan.
I have found that it is beneficial to use double grate spacing between pans in order to help compensate for the baffling, as well. The additional space between the pans allows for more heat to travel inward towards the next pan as it passes above the sides of the first pan, and so on.
I guess looking back through this, try to remember that going vertical and loading it up really changes alot of what is going on in the smoke chamber, not only the flow of heat/smoke, but the amount of exposed surface of the rib slabs if in racks. You have a lot of potentially usable grate space and overall smoke chamber dimensions, and if multiple pieces of meat are smoked while spaced correctly on the grates (chamber wall to meat gap, meat to meat gap), you will achieve the best results based on the design characteristics of this type of smoker. Once you start to crowd things more and more, you will see the resulting finished product quality become more and more difficult to control...to the point where it is no longer controllable and quality deteriorates drastically, unless counter-measures are implimented.
Just some things for you to consider during your quest. I had a very similar idea for smoking 10-12 slabs of loin backs for a gathering a while back, but realizing what I was asking of my smoker, I dismissed the idea. Heat rises up though the vertical smoke chamber very well, if not obstructed. Large objects create obstructions...overcoming the issues which followed from there brought some interesting trial-by-fire smokes in my outdoor kitchen, and the humbling realization that, as good as my plans and efforts were, I could not fully conquer the issues that lie ahead.
Not saying it can't be done, it's just very difficult, at best.