Make templates from cardboard....get some 1'1/2" to 2' thick closed
cell foam.....put templates on foam and cut with sharp knife (or single
edge razor) cutting slightly larger than templates so they will
compress when installed and make snug fit.....make small slit and push
poly rope thru.....make 2" square thin plywood (or metal) "backing
block with hole for rope....put rope thru backing block and
knot.....use long enough rope to connect both cowl plugs.....not fancy
but cheap and they will work.... not hard to do. Or you can buy set
from Sporty's.............

A tip.....make connecting rope long enough to wrap once around
prop....so that plugs will be pulled out if engine is started with
plugs in place.....don't laugh, several years ago I made set of above
described cowl plugs for club owned Champ. A major airline captain club
member missed the cowl plugs on preflight (if he even did
preflight?)....took off and flew for 1/2 hour with plugs in
place....enough to cook and ruin the cylinders....after
overhaul/rebuild of engine we made connecting ropes and policy to wrap
the rope around the prop