Most aircraft engines are air cooled as this saves weight, no coolant, reduces complexity, less hoses and increases reliability. But air cooled engines also have their own set of peculiarities. They need to be kept within certain temperature limits or else the cylinder heads may crack. With liquid cooled engines temperature changes and air flow is much less of a problem.
Before you begin to create you own baffles for your engine it really pays to go around and look at other similar aircraft engine installations. This may take some time but will give you ideas on how to do it, and while your at it: make lots of photo's for reference later on.
This will be of great help later on, possibly years, when you are working on your own baffles trying to get them installed.
You will need enough thin aluminum sheet: 5052-H34 or 6061-T1 will do just fine as 2024-T3 cracks too easily, a set of aviation cutters (left, straight and right types), a drill with rotary file for the larger holes, a set of files, cleckos and rivets, some pencils (sharpie in different colors) and a bucket full of patience.
Create your templates with paper or cardboard, properly identified of course, so that when the time comes to lay them out you can get the most out of the sheet of metal you have without wasting too much material.
Use the aviation cutters, or your preferred cutting method, to cut out the baffles and make sure to stay outside of the lines. The easiest way is to start with the four side baffles, they should fit the cylinder heads and form the basis. Make sure they get a good fit around the cylinder head covers, using a rotary file in a drill works perfectly for this job.
You will need to use a vise and bending blocks to precisely form each and every bend, make sure to use a large bend radius or else you may risk a crack later on. At each intersection of the bend drill a stress relief hole.
Overlap each baffle with the incoming airflow, so that the air enters the pressure chamber streamlined and not turbulent. This helps in proper cooling of all cylinders. When riveting baffles segments make sure to think ahead, so that when ever you need to remove a segment it is not permanently attached to some other baffle leaving you to remove more than is really needed.
When all segments are installed you need to trim the top of the baffles so that it follows the inside of the top cowling with room to spare to install the baffle seals. This will take a lot of time where you will remove and install the cowling several times, seemingly without end.
The baffles need to have a gap with the top cowling of about a 1/2 to 3/4 of an inch so that the baffle seal can be installed. You will need to install the top cowling and feel with your fingers through the front opening making sure that there is enough room for the seal to be installed, if not retrim the baffle with a cutter. Allow the seals to overlap each other by an inch or so which helps air flow.
Install rivets every 2 inches, punching holes in the seal first and use a large washer under the rivet to hold the seal. You can also use strips of aluminum to hold the seals into place. But that will mean more work for you.
Drilling holes in this silicone type baffle seal is really frustrating. It will wrap around the drill bit making punching a better method of doing this. You can use the same tool as you would use for punching holes in your belt.
When all seals are in place and after the top cowling fits perfectly, the time has come to close all remaining gaps in the baffles and seal them with a high temperature sealant (650°F or higher). Use the red silicone sealant from Dow Corning or other suitable brand that you can find. Any A&P should be able to advise you on such a product.
Finally, when its all finished you can anodize all baffle segments in a nice blue, red or gold finish for corrosion protection. It looks great, even more so when you have built yourself a pressure chamber with its own separate top cover.