Riolu(285)/Aron(330)/Ditto(288)/Smeargle(250)/Sableye(380) all seem like good filler, sticking below the average required. With those five pokemon, you have enough room left for Kyurem-B, and those five can function well. Smeargle can put something to sleep, play down entry hazards, and rapid spin them away for only 250BST, Riolu can destroy teams without faster priority/magic guard. If the opponent is dumb and lacks a poison, flying, steel, or poison heal, an Aron can sweep a whole team with only Endeavor and Toxic Spikes support. Sableye is great in OU and can wall most phisical attackers with priority Will-o-Wisp and Recover, alongside almost no weakness (Scrappy Fight-type moves hit it for super-effectivem but those see little use. I could also see CB Sand Rush Herdier getting some use. It has a decent 80 attack and can reach 480 speed in a sandstorm while giving you more BST to work with for your other pokemon.
Hippopotas could help Aron, getting sandstorm so the pokemon immune to toxic spikes (bar Aerodactyl, Gliscor, Gligar, Archen, Aerodactyl, Landorus, and Steels) will still fall, althogh that is bad for Riolu as it shortens Riolu's longevity. Vulpix and Snover could also see use as they give your other pokemon sun or hail respectively, to work with, while not exceeding the magic number for BST in this metagame, 383, unlike Ninetails and Abomasnow. Rain suffers because of Politoed's high 500 BST and a lack of a lower BST pokemon with Drizzle.
Note that Zweilous has too high of a BST to be a filler, although it still has a relatively low 420 BST while hitting within 90% of Kyurem-B's attack and having decent defences with Eviolite. Lairon can also be great, winning most matches 1v1, although there can't be entry hazards and it has a 430 BST, making it too high to be filler.
Note that even though they are LC pokemon, Pontya, Archen, Wailmer, Porygon, Onix, and Lickitung have over 383BST, so they will reduce the average BST of the other pokemon on your team, although I don't think any of them will be useful in this tier. Furthermore, Delcatty, Sableye, Mawile, Corsola, Spinda, Farfetched, Unown, Luvdisc, Delibird, Ditto, Smeargle and Shedinja are all fully evolved pokemon.
Also note that Pickacu has only a 300 BST and can hit 458 Attack or 436 Special Attack when holding a Light Ball, although the fact that a light ball is needed to reach those high offensive stats also means that Pickachu's 35/30/40 Defenses really hinder it as it can't hold an eviolite and those stats can't be boosted by any other hold item, so it is best. Furthermore, its only good moves are thunderbolt, signal beam, hidden power, Thunder Punch, Brick Break, ESpeed (Gen V event move), Fake Out (egg move from Gen IV onwards, incompatible with ExtremeSpeed), and Focus Punch (Gen IV TM more, incompatible with ExtremeSpeed).