ITT: Design an OU/BL/UU viable Mega for a Pokemon with a BST of 450 or less.

Here's a list of most of them, barring things like Volbeat, Lunatone, early route rodents, and Azumarill (but you can still choose them if you'd like).

Sandslash
Venomoth
Seaking
Granbull
Cherrim
Liepard
Fearow
Noctowl
Persian
Kecleon
Arbok
Wigglytuff
Quagsire
Qwilfish
Swellow
Pelipper
Marowak
Sunflora
Chimecho
Swoobat
Wormadam
Mothim
Mightyena
Castform
Dunsparce
Masquerain
Chatot
Vivillon
Sudowoodo
Magcargo
Parasect
Dugtrio
Butterfree
Beautifly
Ledian
Ariados
Kricketune
Corsola
Delcatty
Farfetch'd
Delibird
Luvdisc