We decided to stick with Eheim so all the cannister filters used interchangeable parts. While
Eheim came out with a new 3e model, it didn't seem to offer features I desired for its cost. Instead I ended up purchasing a pair of Eheim Professional II 2026's that were discounted online.
The bulkheads I purchased had 1" slip fit connections on the bottom and threads on the top. They had matching inlet cages that would thread into the intakes, and with a PVC adapter, it was possible to fit adjustable spray nozzles for the returns. Also shown above is the design I arrived at for connecting the bulkheads to the lines for the filters. I briefly considered plumbing it with hard PVC to the filter then using a short bit of tubing to connect the two, but this seemed like it would be more trouble than it was worth.
Here the parts have been glued together and installed. I had to enlarge the opening in the back of the stand to have sufficient access to operate the ball valves. The one has a screw plug installed because I am going to route the output of the filter to the garden area above and behind the aquarium and let it run back into the tank.
I did think later that my choice of the 1" to 1/2" reducing 90 degree elbow was a mistake. I have permanently artificially restricted the flow to 50% what it was. If at some point in the future, I increase the size of the filters or otherwise need more flow, there isn't much I can do about it without replacing the bulkhead since it's all glued together. I should have instead plumbed it to a 1" slip to threaded connection then used a threaded reducer to fit the hose barb. Then I could have easily replaced the reducer as necessary in the future.