Neither.
The problem lies in what you intend to do. If you simply pour the foam onto the exposed seat and then add a layer of GRP over the top you have created a very strong structure with unpredictable flexibility. This will affect the way your kart handles. (They make aircraft and vehicle parts that way, the skin of GRP makes the whole thing into a braced monocoque construction). Besides, pouring foam onto the GRP makes it very difficult to remove if you have made a mistake.
You can put a plastic bag (Bin liner?) into the seat and pour the foam into that and sit in it, but the problem then is that the foam can actually lift you in the seat (my seat insert lifted me about a centimetre in the seat even though there was no thickness in the actual floor of the insert, but it is great in the wet!) Obviously once the foam has set (That's you sitting in the seat for an hour) you can trim the edges off to look tidy.
If you are simply trying to take up space in the sides of the bottom of the seat use thick flexible foam. I have used kneelers from the local garden centre about 20 mm thick,(very cheap) and also neoprene supplied by a kart manufacturer (very expensive) You can cut it to shape and build up layers from thinner foam gluing the layers with contact adhesive.
Use duct tape initially to check the pads are right, then use a contact adhesive (not superglue) to stick them to the seat. If you want the pads or insert to look more professional, cover with material (there's a reason that Tillet use short piled 'carpet').
From experience, you will probably need several attempts, what feels comfortable sitting still turns out not to work while racing.
Also consider the alternative. Get the seat tight enough to hold your hips snugly and use the seat bars to pull it out wide enough to fit your chest.
If your problem is pain in the side of your ribs, hence the rip protector, this is caused by you sitting forward when you go into turns (often because you are bracing yourself to stop your hips sliding around) and the top corners of the sat driving in to your ribs. Fit 'ears' to the top corners of the seat ( I have used rear number plates bent over heat to curve in about an inch and then covered in a layer of foam) held in place by the normal countersunk head screw and nut or even mushroom tape. Not pretty but very effective.) Tillet used to sell the pretty version as part of their rib protection system. Sometimes they need trimming so they don't interfere with your arms or dig into your hips. Again cover with material to look professional
There are lots of other variations. One seat I saw had the gel packs from ski boots fitted, uncomfortable when first you sat but then they conformed, another had an inflatable air pack.
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Custom seat fitting/foam by 'hetherip' (20 Jun 2014 @ 18:03)
