Since on the newer Spyders with the 1330 engine assy, the engine profile is different (which takes up less room below where the tank fits) & the tupperware covering is larger/wider to accomodate the wider engine (which provides more room around where the tank fits) I really doubt you'd be able to shoe-horn anything much larger in there on the older smaller Spyders. Besides, I had one of the very experienced & capable local Long Range Tank builders look at my 2013 RT & for a cost of about $1K they still doubted they'd be able to fab up a bigger tank that would safely hold anything much more than about another 1/2 litre (1/4 gallon?) in the free space available around the existing tank, even with the acoustic panels & that bloody ginormous intake ducting removed, so I doubt you'd have any better luck on a 2012 or earlier!!
If you really need more on-board fuel, I reckon you'd probably be better off looking for an Auxiliary tank or one of those bolt-on or drop-in 'buddy tanks' or whatever they call them; altho maybe to do that you'd hafta sacrifice some frunk, trunk, or pannier space to get any reasonably useful amount of fuel in!! Saaay an extra 5 litres/1 Gallon or so in a pannier or taking up the bottom third of the frunk would probably be useful; it shouldn't be too hard to plumb it in with a transfer pump; & probably only a little harder to plumb it in with a direct change-over switch.
Me, I settled for carrying a 4 litre container in the LH pannier; I found one locally that fitted under the (vacant on my 2013 RT) battery box & still left the outside half of the pannier free for other stuff, so there's not a great loss of luggage space. Over to you :thumbup: