i have made caps for wheels and wheel centers, such as on rallye wheels, several times from aluminum round stock.
depending on the application, they have been retained with o-rings, or V or round shaped retainers that bolt to the back side of the centers [for wheels] or epoxied into the rallye wheel type center.
i realize not everyone has access to a lathe or has a buddy with one, but just throwing this out there as an alternative.
i just made some centers to replace the loose plastic caps that were on a set of chrome plated "centers" that has holes for the [chrome] lug nuts to pass through, and adapted them to some slotted factory wheels that came on my junked 95 caravan.
i'm thinking these centers came on a late model [89-90 ?] dodge magnum. they were retained to the original wheel with three 1/4-20 self tapping bolts, even though the center has five lugs evenly spaced between the lug nut holes.
to mark where these retainer bolts would be drilled on the caravan wheels, i installed three pointed set screws into the center, put a dab of white paint on the points of the screws after i installed the set screws backwards [with the allen key hex into the centers], then carefully lined up the lugnut holes on the wheels and allowed the centers to just touch the wheels, allowing the paint on the screw tips to mark the caravan wheels when the centers were lowered to the wheels. lifted the centers and drilled the holes to 5/16" to allow for some slight adjustment of the lug nut holes.
the description of this procedure is not as difficult as it sounds, and is easier to do than one thinks. rallye wheel centers [early and late style] can be added to factory plain steel wheels the same way.
as the chrome on these centers was bad as well as the center caps missing, they were blasted, primed, and painted after the aluminum replacement center caps were epoxied into place.
as my caravan has the medium gray plastic [wide] side body moldings and bumper covers, i sprayed the wheels a tractor paint gray [massey-ferguson ?] and the centers a darker gray.
i then installed thin stainless trim rings [think 40-50's cars of most makes had these rings] and with the chrome nuts [which are the real deal and not those with the tin caps] makes for a very different look for a caravan wheel.
i also have "big and little" tires installed to make the "bus" look like a "racer".