Keep the current sleek system.
But have weaker alternatives.

Shield as a spell which all mages learn.
With some or all of below weaknesses.
1. Can be dispelled.
2. Have chance of failing. If fail, no DR.
3. Does not work against spells.
4. Uses mana every tick to maintain.
5. If DR kicks in, mana will be used. i.e. instead of subtracting from hp, subtract some % of the original damage from mana.
6. Does not stack with the real 'shield' spell.

Mage cannot have it up all the time (Pt 4 and 5)
Enemies cannot use hit-and-run easily (compare to timer-based idea).
Since they do not know the amount of mana used.
But if they hit the mage hard enough, the shield will fall.

Aura as automated quest reward.
Once player reaches some rank, the guildmaster will give randomly pick a quest from a pool. If quest completed, weaker aura wand will pop for the mage at some known location. Mage can gather the wand as per the normal sleek.
Wand has some or all of following weaknesses.
1. Can be dispelled
2. Shorter duration
3. Fewer charges
4. Prevents recall and teleport
5. Cannot be cancelled
6. Does not stack with the real 'aura'

So if mage uses it, he has real risk of unable to escape if fight goes badly.
Enemies can exploit the known movements, as mage has to gather more frequently.
Quests should be designed to guide mages the proper skills of exploring. (e.g. look at non-obvious places, fight tough mob.) So that they can find the sleeks themselves, instead of relying on the weaker versions or OOC list.

Barrier remains as sleek-only.

So player who cannot or will not find their sleeks for whatever reasons have alternatives and feel less frustrated.