Jason,

If you really want to keep the cost down, try making your own plug out of balsa wood. Local hobby shop sell 4x4x36 inch balsa for $12 here, then shape it( to make it wider just add balsa sheets to the side and use probond ( sandable) Then use fiberglass or carbon cloth and coat with fiberglass. This process will take u some time but it will keep the cost down ( about $60 plus your time)

I did this for a friend back in the 80's, made the hull for him and he made the deck from basswood then coated with fiberglass no cloth.

Total cost was about $140 just for the Sub.( 4 feet long)

Hope this helps you and best of luck.

Mike Holbrook