mid range tank

Hi I'm looking to buy a mild steel a mid range 12-13 litre tank for my gp running an RT225 with a 28 phbh, I'm only on a budget for the pressed lambretta style tank and cant stretch to Diablo \ Oiltek. are there any that fit better (or worse) than others and what fitting problems may \ will I encounter, are there any that should be avoided ? any recommendations greatfuly accepted
cheers
Adam

cheers
Adam