Easy peasy.
When I drilled my WSMs, I placed the hole on the opposite side of the vent. I taped up the area with blue painters tape and marked where I wanted the hole. That location was determined by measuring from both side of the handle bracket to the rivet of the vent.
Once you have located where you want your thermo, use a small, sharp bit to start your hole. Use a variable speed drill at a lower RPM as it helps to limit chipping. Once you have your pilot hole, use a step bit until you reach the diameter hole you need. Don't force the bit, let it do the work, again at a lower RPM.
Once the whole is drilled, I hit the exposed steel with a little high temp black spray paint. I also like to "cup" a couple of washers to match the arc of the WSM and put one inside and one outside and mount the thermo through them too.
YMMV.
Good luck