Do it yourself, safe a bunch of cash and learn something new
The easiest method is fiberglass over rigid foam (closed cell pink blue or green from the builder supply stores). Sheets come in different sizes and thicknesses. I like pink in 1/2" to 3/4" thick for seat bases and 2" for making the humps or tails.You need a glue that is compatable with foam and I like Gorilla brand but construction adhesive (sold in the same area as the foam) works.
Start by laying out and shaping the base. Build up the hump with a couple or three (4) pieces of the thicker stuff. Shape to your liking with rasps, files, sandpaper saws..whatever you feel comfortable with. Sand the final shape as smooth as you can with fine paper. You can fill and sand any divots with drywall mud / plaster of Paris. As the foam is cheap you can make several models to get the shape you like best.
Once you have the shape you can think about glassing. You have a couple of options but you need to know one key factor. Foam will not stand up to common fiberglass resin but is compatable to epoxy (2 part) resins. Epoxies are quite a bit more expensive but easier to work with in these projects IMHO. You can use standard resins but you will need to encapsulate the form first. No resin can be allowed to get to the form or it will eat it. One method is to paint the form with a latex based paint, several coats would be recommended. Another method is to use packing tape and wrap the form.
As far as cloth you would want the finest weave you can find. You can use a course cloth for the first layer and a finer one for the top but it is really not necessary and one fine layer is plenty strong (as the foam provides the rigidity). Cut pieces to shape. Usually one for the seat surface to wrap around the sides and be fixed to the bottom. One for the bottom and maybe a couple for the hump. You will need some push pins to hold things in place.
Now when dealing with glass and resins you need to wear gloves and masks and old clothes. Round up some dollar store brushes and a couple of plastic containers (bottom cut off pop bottles). It also helps to have some type of stand made (a couple of piesecs of scrap lumber) to affix the form to while glassing. Cut open a garbage bag and tape to cover your table. Resin get everywhere.
Start with mixing some resin as per the instructions. Just mix a couple of ounces enough to coat one piece at a time. I find a shallow tray or bowl handy to "load" the cloth. Pour the resin in and dip the cloth in. Make sure it is all wet and there are no dry areas. Pin the cloth in position. I do the whole top side one time and when hard, complete the bottom. When both sides are hard sand off any lumps and paint on another coat of resin alone.
Once the form is completed sand again to remove drips or ridges and then fill any depressions with standard Bondo or the like. Sand until smooth and primer, sand surface fill etc before paint.
As I mentioned before, I find working with the epoxy easier as you have no fear of destroying the foam. You can cut into the form after forming the shape to make holes for lights or glassing in nuts or bolts for attachment points. if you make any holes seal them with some resin painted on, cloth is not necessary.
Depending on the cost of resin ($30/40 a pint maybe) and glass ($5/10 for 5 or 6 sq feet or so)and foam ($8/20) you can make at least 2 if not 3 or more seats.
This technique is also good for making custom parts like side panels and fairings.
The first efforts may take a little time but in my opinion it a technique well worth learning and very satisfying when you get it completed.
Give it a try.
Cheers,
Spyug.
This is the fiberglass over foam seat for my XS Street Tracker. Hope it gives you an idea.
