Senjaz

I would simply make it a multipower with a focus limitation downgraded for the availability of the focus.  Again, this is how I would do it, other GMs might have different ideas.
 
example:
15 multipower (Weapon spells) (30) OAF: Any weapon (downgraded to -1/2) Requires a roll 13-
1u Ice Rune: 3d6 Entangle no range -1/2
1u Fire Rune: 6d6 Blast No knockback -1/4
1u Lightning Rune: 1d6K RKA +4Stun Mod No range -1/2
1u Thunder Rune: 6d6 Sonic Flash No range -1/2
1u Pestilence Rune: 3d6 Con Drain 

Right now, I am leaning towards a multipower where slots are all bought 0 END but there is an END cost to switching slots. I am thinking that the reserve will also be bought with a limitation of slot only remains active for 1 minute, though not sure what price I would give that - would depend on how often I thought that a player would want the slot to continue for longer than that...five turns is a long combat in my games and that would mean the player deciding not to switch to alternative enchantments in that time...