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I would use Bathroom silicone sealant for sealing the shower base to the sides .

Bigg-robbs idea of filling before fitting sounds like a good plan to me .
If you want the tray inplace first , drill some holes from underneath that can be used to apply the low expanding foam thro . ( drill holes before putting shower tray inplace , so you don't drill thro it ) . Afterwards refill the holes if necessary .

atb

Wizard

I have recently replaced the ABS shower tray on our 2002 Eccles with a GRP one. The original tray had a piece of 12mm blue styrene foam as the support for the standing area.  This was reused and stuck to the underside of the tray with non setting mastic as well as mastic on the floor to stop it moving.  This allows the tray to be removed in the future.  I wouldn't use Sikaflex.  The edges were then sealed using silicone seal.  If you cannot get styrene, you could consider expanded polystyrene but this may crush over time.

You don't say what the tray is made from but ABS tends to deteriorate when some chemical cleaners are used on it (same reason mouthwashes are a no-no with plastic sinks) and I'm not sure how expanding foam would affect it in the long run. 

Mike