Sorry to resurrect an old thread, but I thought I would share my summing approach, and see if others have better ideas. For our upcoming gig, the FOH crew can only provide a single mono XLR, so I am building a set of mono-out presets.
Setting the output block to Out 1 or Out 2 works fine, but I often test my presets with headphones which remain full stereo in this situation. The best solution for me to hear the end result in headphones has been adding a mono block at the very end.
Through trial & error, I found that the Jewel compressor block can be used for this purpose. It consumes a relatively small amount of DSP and still sums to mono when the mix is set to 0% (see image below). Optionally, you can drop the volume slightly to account for the +3dB summed signal, but I often do some boosting at the end anyway.
After adding this block, I recommend testing your preset for any weird phase issues. Effects like the doubler, stereo chorus, etc. can sound bad when summed. In some cases, you will need to disable those effects or use single-channel stereo (L or R) instead of summed mono.