Standard method would be to build a birch flat back panel and an angled face panel, with stilts to create the desired angle, cover it in fleece and then apply fiberglass resin to make a hard shell. Sand, apply body filler, sand and vinyl wrap. Alot of work to obtain a difference-making angle. Just a wedge is unlikely to make enough of a difference. Then you've got something that protrudes and is then prone to getting beat up. Either change out your speakers with ones that feature adjustable tweeters or better yet supplement what you already have with a pair of independent tweeters that can be strategically located and attenuated if need be to come into balance. You can elevate the tweeter position vertically but do not stray horizontally by any margin. $50 will be your only investment.