A thumbpick can help turn a choppy and dull finger-picking style into a fluid and energetic one. Unfortunately, thumbpick options are limited. Most music stores typically carry one or two kinds that are often too stiff. However, music stores always carry a plethora of regular plectrums in many thicknesses and materials. Look through the selection of regular picks until you find one that has the right flexibility for you. Then, convert that regular pick into a perfect thumbpick.
Slide your metal thumbpick onto your thumb. Along the midpoint of the pad of your thumb, make a mark across the thumbpick. This mark should be about a third of an inch from the tip.
Cut along this mark with tin snips, removing the tip of the thumbpick. You probably will have to partially unroll the thumbpick to fit the tin snips along the line you have marked.
Score the area of the thumbpick that touches the pad of your thumb. Do this with a flat file. Roughening the metal ensures the glue will stick to it properly. Then roughen one side of the regular pick with the file.
Put hot glue or some other epoxy on the rough side of the regular pick. Then place the regular pick on the thumbpick, rough side against rough side. The regular pick should be oriented so that its pointy end is sticking out in the same way that the original pointy end of the thumbpick pointed.
Pinch the regular pick against the metal until the glue is dry. Roll back the metal so that it fits around your thumb. Slide it on your thumb and play.
Use a sidewalk to score you picks if you do not have a file.