Just read an article about a bomb to summon a mecha knight. I had a similar idea a few days back except I came up with a whole system to balance.
First off, it would be a new set of weapons, wrenches. They would have a longer charge time and would slow (or negate) the ability to move while charging (whichever would feel more balanced). Each wrench would have a weak melee doing damage as though all enemies were resistant to it (damage in the 7-18 range) but upon a full charge would construct something. There would be separate wrenches for building different things and the star rating of each would reflect the strength of whatever is being built. There could be a wrench for a mecha knight and a mecha turret as well as wrenches for the gun puppy series (1 shot and 3 shot separately) or whatever else the dev team sees fit. Each wrenches design could in someway resemble the construct being built
Furthermore in order to make use of them one you would have to swap out your shield for a special item:
1-star "Bag of Bolts": allows the user to build one construct
3-star "Pouch of Parts": allows user to build two constructs
5-star "Tinkerers Toolbox": allows user to build three constructs
These would be overall (i.e. Bag of Bolts allows one construct, not one construct per wrench carried). In the event that a knight tried to build a new construct and go over the limit the oldest construct would be destroyed automatically and replaced by the new one. If at least one of these was not equipped then the wrench would not be able to produce any construct.
All of these items would not actually serve as shields and when attempting to shield would always show a broken shield. However, they would still allow for shield bumping of enemies to prevent anyone choosing to be a Tinkerer from being swarmed and trapped and automatically doomed to die.
Therefore, this concept would become more balanced by both sacrificing a knights ability to inflict damage by themselves as well as sacrificing their defense.
In order to further balance should this still seem too powerful I was playing with the thought of each wrench having a chance to damage self while charging. Unlike being hit by an enemy it would not reset your charge but simply inflict damage. The more stars the wrench has the greater the chance to inflict damage on self would be. In order to negate this a knight would be able to purchase helms specifically aimed towards a Tinkerers needs. These would offer decreased chance to damage self but would offer much worse defense benefits than standard helms. Such helms could resemble safety glasses or a welding mask etc.
Furthermore, the charge time on wrenches could be extremely long making them unreasonable to use frequently. This could be negated by a set of armors that would decrease the charge times significantly. Like the helms, these would offer worse defense bonuses than standard armor but would aid in the act of specifically building constructs.
In this way, in order to actively build constructs effectively, a knight would have to sacrifice his/her ability to defend themselves, putting them at the mercy of his/her constructs and/or party members. This would add an interesting aspect to the game whilst preventing the over-powering of the ability to create such constructs.
I know this idea is not perfect and has been discussed before, however I feel like it was never given proper thought towards balancing. I hope my input at least merits significant discussion by other forum members if not the dev team.
Look, just because you have an interesting idea doesn't mean that whatever game you happen to be playing at the time should implement that idea. Good games have focus, and their feature set is limited so they can actually be built in a reasonable amount of time. You don't get better games by just gluing on every good idea you can come up with, you get better games by designing a core set of features that all work together and then cutting off the non-essential bits.
What you have presented is really quite a complicated idea, in terms of programming, balance, art, and AI. And most of it is really completely out of character with the rest of Spiral Knights' gameplay. Those parts that are similar to existing stuff in Spiral Knights (like encouraging players to charge up before stepping on party buttons, or choosing all of their other equipment purely to support one specialized weapon) are more extreme versions of the BAD parts of Spiral Knights.
I'm sure an awesome game can be constructed around the idea of creating machines that fight for you. In fact, I believe many such games already exist. But Spiral Knights is not one of those game, nor should it be.