I have finally got my hands on a pair of basic fibreglass shells and I now need to get them trimmed.
RazLady is very handy with a sewing machine and has trimmed boat seats before but these will really present a challenge.

As you can see they are fairly conventional bucket seats but they have a more laid back angle and sit really low in the car - perfect for my low roof.

Is there a 'how to' somewhere that will give me some ideas?
Rescued attachment Ultima seat.jpg

Mark Evan's MG series had a bit on how to trim the MG's seats - as did the Westfield one.

The sides usually get a stiff foam and glued into place then covered with vinyl glued to the shell. The area that you sit in and press your back against gets panel cushions made that are stuck in with wide strips of velcro.
(If you get wet you can remove to dry out)

One of the major seat manufacturers do extra padding for their seats. Cannot remember which one. May be able to copy those american stock car seats that have a clip over cover.
Best to take the mrs to HALFORDS and look how the buckets are trimmed in the chav section for corsa bling to get a good idea.

To cut the foam you make a hot wire and stiff templates pinned to the foam.HOT WIRE LINKY

Hope that helps.

Since you have the option, make the seats to fit your butt as comfy as possible .
Do a GIS for "racing seats" if you want ideas on how to stich them etc. An alternative to using thick foam is to layer the foam. Buy 1" thick stuff and glue it together until you get the thickness/shape you want. You still need to trim it, but its cheaper and you can use different density foams for maximum comfort.
Cheers.

to cut the foam use an electric bread knife. Thats how I cut foam in my work as an upholstery designer. To be honest for the thickness you are going to be using good scissors should do it.

So do I simply start gluing layers and shapes of foam onto the shell until it is the right shape and comfy, or is there a recognised 'standard' way of doing this?

Just a suggestion, glue fabric directly to seat shell to create a 'finish', then create individual seat cushions/pads with foam and cover them.
Velcro the pads where you want them

Thats a good idea, in fact that is how a lot of the aftermarket bucket seats are done. I will have to pad the sides first though, because the shape is too square to be comfy so possibly some triangular wedges in there, a wedge shaped seat cushion and flattish back cushion and a lumbar cushion for my dodgy back

Lining the harness holes should just be ok with some edge trim I guess,

In an attempt to keep it locost foam is available in cushion form from settees currently avaible from a skip centre near you
Seriously.

I've got the same issue with covering my cf seats as the inside is the "raw" unfinished side.

Got some new 8mm thick camping "roll mats" for £2 off eBay which is a good thickness/hardness for the seats.

Also got some nice waterproof and slightly stretchy fabric too.

Still not actually done it though

Funnily enough I have just bought a couple of those myself - time to test my trimming skills