Any tools for easily increasing monster CR?

Mike Shea had a great idea for creating tougher versions of monsters: double it's hit points, add +2 to its attack rolls, damage, and DCs, and give it one extra attack as part of its attacks action. This will make it about twice its base CR. You can also increase its size by one step if you want it to look bigger and badder.

EDIT: I reread this and realized you're asking for a tool. I'm afraid I can't be of any help there. Maybe just put all of the above into the DDB homebrew thing? When I do this in person, I usually jot all this down on a sticky note and put it on the monsters page in the MM.

I have more or less abandoned CR's with the exception for identifying when encounters are going to be vastly beyond the party's capabilities (if the encounter is worth double the Deadly XP level, it's too tough). The difficulty suggested simple doesn't compare to what a party is capable of doing, in part because the moment one or two creatures go down, the encounter difficulty drops massively. I find it deeply tedious to have to roll initiative for a bunch of creatures that I know are going to die within one round of combat, and there's little in it for the players.

My experience with the Encounter Builder tool is:

All encounters of Medium difficulty and below should be considered Trivial. They will rarely last more than 2 rounds at most, and are usually ended by a single spell.
Example: 5 x Level 8 characters vs. 2 Trolls is considered Medium. With only 84 hit points, I would expect one troll to die before taking its turn, and the second to be mopped up halfway through the second turn of combat.

Encounters of Hard difficulty should be considered Easy, and unless they have a hard hitting breath weapon to unleash in the first turn, will probably not cause any significant challenge. They may last as many as 3-4 turns of combat. If the party are fresh and prepared to blow what we'd call 'Cooldowns' in the MMORPG world, then the party will have done sufficient damage in the first turn of combat to have reduced the enemy threats to a Medium or below encounter, and probably just spend a couple of turns using Cantrips and basic attacks to finish the enemy off.
Example: 5 x Level 8 characters vs. 1 Green Slaad and 1 Red Slaad. If the Red Slaad lives long enough to take a turn and manages to hit with all three attacks, it will inflict just 22 damage on average. It is a negligible challenge. Hitting with all 3 attacks, the Green Slaad will deal only 33 damage. Together they'll manage an average 55 damage if they hit with everything against a target with no defences.

Encounters that just push over into Deadly difficulty should be considered the standard - Medium. They will typically offer 4-5 turns of combat, allowing the characters to make significant choices about what abilities to use.
Example: A Mage riding a Winter Wolf, supported by a Young Black Dragon is Deadly for our 5 level 8 characters. The encounter offers some choices about targetting. Of course, the mage will almost certainly be dead before the end of the first turn unless he starts with Greater Invisibility up. There are many ways to make this encounter trivial difficulty of course - Banishment will do it just fine.

A generally good rule is that if an individual creature's CR is double that of the average party level, then the creature will likely have access to spells or abilities that the party cannot handle - a level 8 party cannot effectively cope with a CR17 Death Knight's Hellfire Orb, for instance.

https://1-dot-encounter-planner.appspot.com/quick-monster-stats.html

https://********/crcalculator.html#0,13,1,3,false,Medium,1,10,false,0,false,0,

https://tetra-cube.com/dnd/dnd-statblock.html

All of these website used separate or in conjunction are great for DMs wanting to make or adjust monsters .