A Some What Quick & Dirty Locost Nosecone
If you don't want to go to the effort of duplicating the other nosecone design found on this site or just don't like it, here's a way to roll your own.
The dimensions shown here are for a standard Locost (Book) frame. You can modify them any way you need to fit your frame. Briefly, the process is to cut 1 and 2" thick foam rectangles to the dimensions given; temporarily glue them together; sand the glued up block to the shape you want; disassemble and trace the lines made by the sanding so you can transfer them to the right side; cut to line, glue up and sand smooth.
The drawing here shows the view of the nose's left half from the front of the car.
The form is made up of 13 rectangles of 1 and 2" foam. All but one are 2" thick. Once cut they are glued together with white glue. Just a dab in each corner should be enough to hold it all together while sanding. Remember you'll have to break it apart to make the templates. Too much glue and this will difficult or impossible.
When gluing up you'll want to work on a flat surface. Draw a line on the surface to register the 2" elevation lines to. The 2" elevation line is also the top of frame line. You should mark this line on each foam rectangle perpendicular to the centerline.
I'd start by taking one of the rectangles in the middle of the form and glue the centerline edge to the work surface so it is square to the reference line and perpendicular to the work surface. Then let the glue dry. A little care here and the rest of the glue up will go fast.
Once this piece is firmly in place the remainder can be glued to it, each other and the surface. You can use rubber bands, tape, string, whatever to hold things in place while the glue dries. When it's all dry sand to your desired shape. In the section drawing I've suggested some radii you may want to work to, but it's your foam. If you take too much off you can always glue another bit on. Same thing if you want to add a flare or a blister.
When you're happy with the shape of the form take a hand saw and cut the form free from the work surface. Separate the rectangles and trace the rear edges onto some stiff paper and then on to the right side rectangles. Cut the right side rectangles to the traced lines and glue up as you did the left. You can use more glue this time. Sand the high spots down and glue the right side to the reassembled left. Do whatever clean up sanding is necessary and apply your finish surface. For this you can use plaster or drywall taping compound. The finished form can be used to pull a female mold or a one off nosecone.
If you find this process useful it can be applied to produce parts for the rest of the car which are mirror images of each other, such as, clamshell wings and rear mudguards. HTH