The whole point is not to have to use RDT or specific spells. However conjured barriers using normal spell rules would normally use a range of touch if you were defending yourself. Creating a curved stone wall four paces wide by 2.5 paces high and a pace thick only requires a Level 5 spell if you only need it to last a round
That simple barrier would be sufficient to block nearly any targeted spell simply by rendering the caster unable to still see the target. The actual Fast Cast Spell Defense roll effectively covers what you are wondering if a Finesse roll would be required for. Are you fast enough casting the effect and are you positioning it properly to block the hostile spell.