The easiest way is to add a bump to either the belly or the inside of the cowl. A belly bump should be fairly rounded on the front and extend just inside the cowl, and very gradually sloped to blend back to the original belly shape on the back. There are several recent pictures here on VAF of those that have done this. This may produce a modest speed increase from the reduction of cooling mass flow, but does nothing to reduce frontal area.
A bump on the inside of the cowl should be blended in front and can just end abruptly at the cowl exit. This will likely not improve speed because you will get some base drag on the back of the bump that will offset any momentum thrust you gain with the smaller exit. Might even be slower. But there is one good thing about this bump that I will discuss in a minute.
The very best thing to do is to reduce exit area and at the same time reduce airplane frontal area, by creating a contoured valley in the cowl between your exhaust pipes. A flat section between your pipes can reduce the exit area by any amount you want, and then blend into curved tunnels that wrap around your pipes with some clearance (3/4" or so) for cooling of the fiberglass. This approach offers the greatest chance for some speed increase while getting the cooling you want.
Once you make that reduced frontal area cowl, a cowl flap can be put into the flat area to allow you to increase the opening for added cooling during climb. One question is how much you can actually reduce the exit area. Dan Horton, who had a 4-into-1 exhaust, made a replaceable section of the cowl so with a handful of separate sections, he was able to test different exit areas. With that, he also ended up incorporating a clever cowl flap as well. For cross-over exhaust with 2 pipes, this is not as easy to do. But, you could make a series of bumps to try, attached to the inside of the cowl as described above, that restrict the exit by varying amounts. No expectation of speed improvement at this time, just testing for cooling efficiency. Once you figure out the size of exit you want, then you could replace that with a proper fiberglass valley-and-tunnel modification to the cowl that achieves the same exit area. And keep the option open of adding a cowl flap to it as well.
Bear in mind that as you reduce the exit, you will not only decrease the flow through the engine cooling passages, you will also reduce the flow through your oil cooler (unless it has a separate exit). Most parallel-valve engines have adequate oil cooling to allow a substantial reduction of exit area and still cool the oil OK. Angle-valve engines put a lot more heat into the oil, and so if you reduce the air flow through the oil cooler, you may have problems. This can be offset by improving the flow through the oil cooler in other ways. I am trying to accomplish this by putting a diffuser on the oil cooler exit. Others have ducted the oil cooler flow out through a separate exit, which, if done well, can work very well. Obviously having as little pressure loss in the flow upstream of the oil cooler is best -- if your cooler is mounted to the rear engine baffles, this is about the best you can do. A small improvement would be achieved by putting some radius lips around the perimeter of the cooler to help the flow go into the cooler.
"The Magic Carpet"
Hobbs 525 in 9 years (would have flown more this year if not for fire smoke)
also LS-6-15/18 sailplane
VAF donation Jan 2019