For this application (no heat) we want the mould flexible and durable. We are going to use Mould Max 30. The mix ratio is 100:10 (10% activator).
Before we start the mould, this is what you will need.
 - Mouldmax 30 (available from AMT Composites) It will come with it's catalist. If you have MM 60 it works perfect for this application too but it isn't as durable. MM 60 is 60 shore harness while MM30 is 30. Do not let them sell you Silsmooth or any other product. They do not degass.
- Silicone thinners (available from AMT)
- A kitchen scale
- Something to mix in, make sure this is a container wider in size rather than deep. It will help to expel the air.
- Mixing utensil, I use tong depressors.
Before we start. You need to add 8% by weight (not more than 10% max) of the thinners into the Mould Max 30 and let it stand for 24 hours before use.. This is very important, especially if you do not have a vacuum chamber. This will make the silicone flow better and assist in expelling air.
Try to mix it with strokes and try not to be too vigorous. You have time, dont rush it must be mixed properly with no streaks. The pot life is 45 min.
Method for calculating the volume of silicone required.
Before smearing the the box with Petrolium Jelly (Vaseline)
Mock up the mould box.
Then place some rice or dried beans or anything similar but clean in the box spreading it out evenly at the level you require it, which should be minimum 10mm above the plug.
Then place this in a measuring beaker/jug.
If it comes to 100ml add 18% and this will be the volume of silicone required by weight.
N.B. Water has an SG of 1 and Mould Max 30 has an SG of 1.18
Pour into the mould with a light but steady stream from one corner. then you can follow the flow staying just behind the edge of the flow.
Now you can sit and pop bubbles to kill the bordem or you can go have a cold beer!
Allow 24 hours to set.