I have used 5 minute epoxy to beef up thickness on thin models. In this case perhaps you could put a thin coat of epoxy on the inside of the models, and when set, reinsert the model into the mold and re-pour just the bottom half. You would need mold release for the silicone on silicone, or, let it bond and re-cut the mold. In certain models, similar to this, I have simply abandoned the inside half of the mould and painted or poured wax into it and then poured it out after it forms a skin on the surface. Often many tries to get the thickness and temperature just right. Too hot and you might get uneven thickness on high and low sections of the relief detail of the design, and too cool and you risk bubbles. To solve this problem I have poured a hot layer for insuring a complete coverage, and then a second cooler coat to insure proper thickness. I have also used a paintbrush to apply the wax, which pushes out air bubbles. I use foundry blend wax for this which is a happy medium between the softer “victory brown” sculpting wax and injection wax. However, I think in this case, using the 5 minute epoxy would be the best bet. It self smoothes on the surface. It looks like you would only need to apply it on the parts that are thin.