Any joinery people here? I need some MDF help.

[Aperitif]

My quest is over. James Latham in Hemel Hempstead sell 1220 x 2440 x 18mm sheets of Kronospan Deep Router Grade MDF for £20.87 plus VAT

BTW @Aperitif, having the rear cog/s inside the chainstay is an absolute abomination.

The cog, or drive (D) is always concealed. This is known in the picnic bicycle trade as a 'Formal-D-hide'. Biscuit joints are apposite for sandwich bikes, and help achieve a rich tee joint.
Just because you have got some stuff lined up, don't think we'll forget this 'toff's guide to posh MDF' thread, Martin...