Buy runny silicone in the first place.
thinning it might mess up the curing.
I use some neutral cure stuff at work that is about the consistency of cold engine oil. It comes in a normal cartridge and it actually pours out the standard cartridge hole.
if you make a puddle it will settle and cure just under 1mm thick.
I used it to make a new section of floor in an old fridge. it took about 2 weeks for a 1 inch thick pour to set.
Look for types designed for potting components, these are thin to let the bubbles out.