Edit: while I haven't tried it yet I probably recommend the Z-Butt system over my own design, but I'll leave this up here since it doesn't require as much accuracy in printing the base plate.

This is a mold form for 2-part molds of small objects, in my case keycaps for mechanical keyboards.

The mold form consists of two parts. the bottom has registration built in and a small platform for the object to sit on top of; the top is a square piece that fits onto the bottom mold.

In order to create a 2-part mold for an object, follow the instruction below!

1. secure the object to the bottom of the mold, in the center of the platform, with a small amount of super glue. Make sure that the object is slightly smaller than the platform, or venting the mold will be difficult and will not work correctly.
2. hit the bottom form and object with mold release; if there are cavities on the object make sure mold release doesn't collect in them by brushing the object with a paintbrush.
3. pour whatever mold material you prefer into the mold, and wait for it to cure.
4. remove the mold from the mold form by carefully prying it with a flat object (paint knives work great for this, and you probably have some since you're on thingiverse). remove the object from the platform and place it back in the mold.
5. flip the mold over and spread mold separator onto the top of the mold, making sure to spread some in the registration divots. I just use vaseline/petroleum jelly, it works pretty well.
6. add venting pipes to the object as required. For keycaps I use q tips with the cotton cut off, secured with super glue.
7. situate the top mold form on the bottom mold. there is a flare in the bottom mold that will naturally arrest the top mold form.
8. pour your mold material again and allow to cure, then separate and use as desired.

I am by no means a silicone molding expert, so if what I've written here conflicts with what you have been told then by all means adapt the steps as you see fit.