Is there a way to block during your turn?

Question

I'm wondering if there's a way to get Snarespinner to reduce the cost of Molten Monstrosity by {3}. We can assume opponent has a flying creature. The idea is that with [mystery card/ability], I can get Snarespinner to block opponent's flying creature, and then cast Molten Monstrosity for {3} discount. I'm only interested in solutions where I cast Molten Monstrosity on my turn, at sorcery speed during one of my main phases.

Relevant text of cards:

Whenever Snarespinner blocks a creature with flying, Snarespinner gets +2/+0 until end of turn.

and

[Molten Monstrosity] costs {X} less to cast, where X is the greatest power among creatures you control.

Answer 1

I don't think so. Comprehensive Rule (CR) 506.2 sets the defending player to be the non-active player:

506.2. [...] During the combat phase of a two-player game, the nonactive player is the defending player.

and CR 509.1 allows only the defending player to declares blockers:

509.1. First, the defending player declares blockers.

Therefore you cannot declare a creature as a blocker during your turn.

(References are to the 2023-04-14 Comprehensive Rules).

Answer 2

Donate your Snarespinner at some point, then on some subsequent turn, cast Academic Dispute on the Snarespinner to force it to block. Attack with any flying creature, then after blockers are declared and the Snarespinner's trigger resolves, activate a Homeward Path to regain control of the Snarespinner. It will have the buffed stats from combat, but you can now use it at sorcery speed to discount your Molten Monstrosity.

Of course, there are better ways to get the discount, but this is one way that meets all the requirements you set out, with the important exception that your opponent blocked with the Snarespinner on your turn, not you.

Answer 3

There are three cards that cause creatures (tokens or cards) to enter the battlefield blocking.
The commonality is that they all rely on creatures attacking you or creatures you control blocking. In other words, they all heavily check that you are not the active player.
This is furthered by the rule:

506.3d If an effect would put a creature onto the battlefield blocking but the creature it would block isn’t attacking the entering creature’s controller, a planeswalker that player controls, or a battle that player protects, that creature does enter the battlefield, but it’s never considered to be a blocking creature.

Note that these can't cause Snarespinner's ability to trigger, since the blocking creature is never declared as blocking.

If you are open to using un-cards or test cards, you can block outside your turn if an opponent plays with Party Crasher or Throat Wolf.

Answer 4

If the goal is to literally have Molten Monstrosity cast on the same turn as you block with Snarespinner, all you need to do is give Molten Monstrosity flash, such as with any of the following:

• Vedalken Orrery
• Teferi, Mage of Zhalfir
• Emergence Zone
• Savage Summoning

(I recognize the question specifies sorcery speed during a main phase, which this is not)

Answer 5

There are a few silver-bordered cards that would allow or force a creature to attack a player on their own turn, which would allow that player to block as normal.

• Evil Presents lets a player put a creature into play under their opponent's control, and forces that creature to attack their controller on that player's attack step.
• Party Crasher can attack during opponent's combat steps, which would allow him to attack the active player.

Being silver bordered and thus illegal in every constructed format makes them somewhat useless in reality, but they do exist.