what aftermarket fuel tank do people use

[Henry]

I used a 35l boat tank in mine. Run the fuel limes out the can through the hole for the filler pipe. It passed all it's mots. Just make sure it's bolted down.

[Rhinoman]

The IVA manual would seem to indicate that even on a 'new' car it would have to be from a production car or type approved:

IVA Manual states

Note 2: Plastic Fuel Tanks
Check that the fuel tank:
Is unmodified and an original fitment in the case of a mass produced vehicle or appears to have previously been fitted to a mass produced vehicle or is accompanied by satisfactory evidence provided by the tank manufacturer or approval authority showing that it has been manufactured and tested to meet the standards required for road use.

That would mean that it must carry the appropriate 'E' marking. From what I've found so far plastic tanks are only legal on vehicles made after 1993.