I would want some new ideas for a split mould. My last mould was leaking between the flanges. I am going to use resin infusion, so its critical that there is no air leaking. I included a photo of the part. It will be a three part mould. I will make splits above the door. Second photo has one idea I got, that the flanges would have 90 degree angle at the end, so i could put vacuum tape on it. blue and white line are different part of the mould. black line is plug. Top of the blue and white line is the vacuum tape.

As for the silicone gasket, I just use an automotive product. Same stuff used to make inlet gaskets etc. This is the easiest way because you can just build your mould as per normal with no special considerations. Then apply the silicone to one flange, making sure you encircle the bolt holes, and then bolt the mould pieces together. It's important to do this while the silicone is wet and hasn't started to set up. The silicone will squeeze out to virtually nothing, so you can get the same split accuracy as you would without it. Be aware that if you apply a huge amount of silicone, you may get it squeezing out the parting line and onto the mould surface. So you don't want too much, just a small consistent bead.