A stack of 1" diam abrasive discs are sold in a tube wherever Dremel tools are sold. These black ones are .020", and the brown ones are .032" if I recall:
from here: https://www.google.com/search?biw=1139&bih=767&tbm=shop&ei=lf64XZuOBsHwsQXUmZ24DA&q=dremel+cut+off+bit&oq=dremel+cut+off+bit&gs_l=psy-ab-sh.12...16342.23108.0.26229.12.12.0.0.0.0.121.1321.0j12.12.0....0...1c.1.64.psy-ab-sh..0.2.218...0i13k1.0.YNe2jgwzPE8#spd=14050369673805513113
I use different size combinations to get the slot size I need.
One secret is using a "steady rest" next to the vise holding the dremel. This supports the weight of the gun and also provides a surface to rest your hands on while guiding the sight base into the cut off disc. A board clamped to your work surface works well. You need scribed lines on the sight base to follow to get the groove centered in the sight base. The main secret is to use gunsmith glasses to be able to see that the disc is aligned with your scribed lines Use your master shooting eye and cover the other lens with tape. You have to get too close to the work for both eyes to focus that close. Most importantly; you need a little practice.
A gunsmith will of course use a milling machine which eliminates the required skill. I have a milling machine but the set up time for these small tasks will kill ya, and then there's buying cutters. The time required for set up is what drives up the gunsmith cost. And you won't find any with the skill to cut with a dremel.
After you get the depth of your groove cut. Run a dressing stone (like used to dress a grinding wheel) against the cut off discs for a couple of seconds. That will square up the corners of the cut off discs then run it in once more to sharpen up the corners at the bottom of the mortise (1/2 moon groove) in the sight base. Or just round the edges on the sight blade tenon that fits in the base groove for a nice snug fit. Clamp the blade in place in the base groove and drill the cross pin hole thru both pieces and install the retention pin.
Any questions, just ask and if you go forward let us know how it worked for you. But most of all have fun!