Have a look here:
http://www.rejel.com/rejel-store/cavity-wax.aspx
I'd personally go with Dinitrol before any other brand. Quality, modern stuff. Volume-wise, if you got one of the six packs of 3125 cavity wax (for schutz gun/compressor) spraying, that would easily be enough IMO and you'd have spare for future top-ups.
Are you going to use an underbody wax too? Again Dinitrol do a great underbody wax - 4941. Much better than the Waxoyl equivalent. Sprays nicely via schutz gun but also really well (very smooth) in aerosol form - which cannot be said for Waxoyl underseal, which is naff from a spray can.
They also do a good high-temperature, clear hard wax - its meant for engine bays and components but can be used as an underbody spray too. Its great. Invisible on components. Would be good underneath a nicely painted shell as an alternative to black underseal (might be a little yellowy where you get thicker accumulations).
