Fast "Generic Monster" Stats

[TeChameleon]

Hopefully this is the right place to post this- bit new to this bit of the Playground.

Basically, I offered a (very busy) forever-DM friend some help with writing up a one-shot scenario, however, my ability at monster balance is somewhere between 'kind of crap' and 'apocalyptically horrible'. Are there any free pre-made monster stat sheets (for D&D 5th Edition) that I can plug in a couple of special abilities as needed and call it good, and where can I find them?

If it's needed, there are 5-6 players at level 3.

Thanks in advance, appreciate it!

[MutantDragon]

Honestly, my advice is to determine how much of a challenge you want for the group, determine what CR that works out to be, and use the tables straight from the DMG for quick monsters. You should be able to whip up something reasonably balanced fairly quickly.

[Yakk]

Proficiency = CR/4 + 2 (round down).

Attack bonus = Proficiency * 2
DC = 8 + Proficiency * 2
Saving throws = +Proficiency, *2 if this is a "big strong monster making a strength save", *0 if this is a "dumb monster making an int save".
AC = 10+Proficiency*2
HP = 40 + CR*10
Damage per attack = 5 + CR*2
Attacks = 2

Using my https://www.enworld.org/threads/ridiculously-simple-monster-building.677070/ to judge this:

(CR/4 + 2)*2 AC points
5 + CR*1.25 HP points
(5 + CR)* 2/3
(CR/4 + 2) ATK stat
=========
14.3 + (3.3)CR

So you'll end up a bit weak.

Add on two extras:
(10 + CR*5) HP
Extra attack (could be a legendary action)
Tactical bundle
(CR) extra HP, 3 Legendary Resists and +Prof to all saves
Make an at-will attack an AOE, with save for half damage.
Recharge 56 encounter ability that either does +50% standard damage (8+CR*3) on (two targets, or save for half on one) replacing 1 attack. I advise rolling on the previous turn and telegraphing it is ready, because that is fun.
Recharge 56 aoe that does +50% standard damage (8+CR*3) replacing all attacks.
Significant resistances/immunities that matter (resist all weapon damage? Resist almost all spell damage?)
Hard to stop "near free" advantage to yourself and/or pack mates
Really nasty special ability (like damage transfer)

A "Tactical Bundle" consists of CR points to spend on:
Magic resist (2)
Flying and ranged attacks (2 under CR 9, free otherwise)
Near-Constant Invisibility (4 under CR 9, 2 afterwards) (assuming you rule Invisibility using a weak interpretation of raw -- if it is auto-hide it is much better)
More AC or ATK or DC (1 per point, don't get crazy)
Regeneration (1 per point)
Lesser abilities that still matter, like parry or the like, eyeball it.

...

This will work better at CR > 2; at low CR the rounding I did will end up causing problems (PCs will die too easily).

But it gives you a baseline to work with.

[TeChameleon]

Thank you very much for the help, I think I've got it sorted now.