In modded MineCraft, it's often the case that we end up needing an empty hand to do any number of things. Be it interacting with the corpse of a dragon we just murdered to harvest that sweet sweet loot, or toggling the state of some machine or whatnot, we'll often find ourselves having to move things out of a hotbar slot (and our offhand slot often enough) to complete these actions.

I'd suggest that to help alleviate this problem, our toolbelt is given the option of two more upgrades:

Holster:

Crafted as follows:
https://i.imgur.com/yZH3lXN.png
(string, diamond, string;
leather, EMPTY, leather;
string, leather, string;)

The first one you add to the toolbelt adds a Main Hand Holster, and the second one adds an Offhand Holster. Alternatively, a recipe using an emerald instead of a diamond could be used for the Offhand Holster, but it doesn't seem to add anything except needless complexity.

Added to the toolbelt via the anvil, and when added pressing and holding either the Holster Main Hand hotkey or the Holster Both Hand hotkey will take the current item in your main hand (be it something you could normally put on your toolbelt or not) and temporarily store it on the toolbelt. After adding two, you can toggle either the main hand or the offhand with those hotkeys, or both at the same time by using the Holster Both Hands hotkey.

If the option to toggle the holster button is enabled, instead of pressing and holding either of those keys you would instead press them once and the item would be temporarily stored on the toolbelt.

If you were to open your toolbelt via the "Swap Tool" key, the main hand holstered item would appear on the user's main hand side as configured via the "Options -> Skin Customization -> Main Hand" setting (or alternatively it could be toggled via a config option if that setting isn't accessible).

It may make sense to also lock the player's main or offhand slot so it cannot be changed if they have something holstered (cannot add anything to the slot that is holstered, cannot scroll the mouse when holstering the main hand, etc), but if this is added it would ideally be able to be disabled via a config setting.

Hopefully the above makes sense and ideally it is also something that sounds like it is both doable and something you are interested in adding to the next release of ToolBelt.

Thank you!