I have used a Catalyst for the last 4 years with a Bearikade Weekender. I typically set the canister in vertically on top of my sleeping bag which is at the bottom. I put my bladder and the "heaviest" of my lightweight items next to the canister to balance the weight distribution, left to right. I also had my tent in the outside side pocket opposite to the canister side. I have averaged 30-38 pounds of total pack weight of which 12-18 pounds of it was food and canister, and it has worked fine on up to 350 mile trips, with re-supplies about every 7-10 days.
I have also tried the Bearikade Weekender in the ULA Circuit, Ohm, and Conduit. In all cases, the application has been the same - vertically on top of my sleeping bag and pushed towards either side with other heavier items next to it to balance the load.
I was able to get the canister in the Conduit, yet, it stretched out the pack so much I lost most of the use of the front mesh area which I need to use. While I was able to get about 9 pounds of gear and 12 pounds of food and canister in there (21 pounds total), the pack didn't ride very well on me at that weight and with this canister. I wanted a lighter pack than the Catalyst, yet I think this application is pushing the limits of the Conduit and my week long time frames in between re-supplies.
I tried the same setup with the Ohm (the next lightest ULA pack) with the same application. The added suspension on the Ohm made it ride better on me yet, my hips were still sore as the canister pushes out the back of the pack a bit on me and lessens the amount of hip belt I have to use. This limits how much of it falls directly on my hips and at 21-23 pounds, it seemed like I would be unhappy with the fit when trying to do 25-30 mile days.
The same vertical application of the canister in the Circuit worked great. It rode as nice as my Catalyst and even weighed about a 6-7 oz less. I just couldn't justify spending several hundred dollars to save 6-7 oz. when my Catalyst is still like new. If I had to buy a new ULA pack today and I wanted to use the Bearikade Weekender in it and I would be carrying about 23-30 pounds, I would get the Circuit. If I was going to push over the 30 pound range and into the low 40's which I have done for several days in the Catalyst, I would buy the Catalyst. Another variable that comes into play is if someone needed the extra space the Catalyst offers.
If I was using the Bearikade Weekender for shorter trips where my total weight might be significantly lower, the Ohm might also work, yet each person would have to see if the savings in pack weight was worth the sacrifice in comfort, if in fact they had a comfort issue like I did.
I hope this helps. Please let me know if you have any additional questions.