There are a lot of technical details to do this right but I will give you a few suggestions. I dont know what the equipment is you have, what is the table size?

The master model/tool should be made of a material that can handle heat like aluminum, aluminum filled epoxy, ceramic, plaster. For just a single copy from a master model the material can be a hardwood with a hard shell like fiberglass. You can damage the master model if the material is soft due to heat and compression from the vacuum.
The surface finish of the tool does not need to be polished in fact a high polished surface will stick to certain materials. Its better to have a lightly sanded surface to allow air flow. The plastc will not transfer the sanding marks if they are very light.

The tool should not be taller than 6in for a sheet thickness of about 0.030in. If the tool is taller then the sheet thickness has to be thicker.
A good choice for your cowl material is polycarbonate/ Lexan (same thing) or ABS Styrene. These sheets in general are about $5-6 in quantity and about 20x18in for example.

For a round cowl oriented vertical to the table you should make a 0.75-1in tall ring that is 0.5-0.75in from the tool on all sides. This will control the sheet and webbing which will ruin the part. Make this ring from a hardwood and tapered so it will release from the sheet.

The base of the tool should have a 0.25in minimum base to allow the plastic to pull past the part. So the part line/cut line should be 0.25/6mm up from the table.

In the center of the cowl on top (front) where you want to pull the plastic into a pocket you have to drill a few 1/16in holes through to the base to draw vacuum. Place these holes in the edge/corner of the pocket to draw the plastic tightly into the detail.

The plastic sheet must be heated to the point when the sheet droops down from the frame evenly. Only then is it ready to suck down on the tool.

Once you get a few parts done to practice you will understand the behaivior of the tool and the machine.

Post photos!