oooh a bag of holding.
Let's take a swing!
I'll start with the MuTe Base 4-- that seems fair, with the change of dirt to something highly unnatural. Working off the MuCo example for Preternatural Growth or Shrinking and the base 4 option to grow to 8 times its size, I'll say it can shrink to 8 times its size. Clearly, this will require some discussion with my troupe, but it's not crazy
As per page 154, the change for metal/gemstone is +2, that gets us to 10 (Base level 4 + 2 magnitudes is 4->5->10).
Range will be touch-- we put the object in the bag, touching the lip of the bag. There's +1
Duration -- we can do concentration, the bag will maintain the concentration when we get to that point. There's +1
Target -- We're affecting an individual, but, we also want to be able to affect, say, a sack full of coins we put into this sack, so I'm going to go with the suggestion of Room, saying that the object must fit inside the mouth of the bag to be affected. The level of creative interpretation here is left to the troupe. That's +2
That starts at: Base 10 + 1 (Touch) + 1 (Conc) + 2 (Group) for a total of 30. But! we do want it to affect more than Terram, so we add Animal, Aquam, Herbam. If we want bones or corpses, we add Corpus, too, I suppose. Whether or not each one adds a magnitude will depend on the Troupe-- some could say that because the contribution is not cosmetic-- the shrinking of all things is part of the point-- we get to: 30 + 20 (4 magnitudes!), for a MuTe 50 base effect. Another troupe might say that we're already accounting for reducing metal and gemstone, we're comfortable with this effect in our saga and don't need to make this harder than it is, leave it at a MuTe30
Now, we can make this a lesser enchanted item, it only has one effect that works while the item is in the bag. We'll have to be able to do it in a season. This means the shape and materials used will matter here. Following the request, we'll make usable 50 times a day, so +6, and it maintains concentration for +5, that gives us a total of MuTe 41 or 61.
The trigger will be touching the object to the lip of the bag and saying "In you go." No linked trigger necessary.
This will require 5 or 7 pawns of vis-- 1 for every 10 or fraction thereof, and since we can only use Magic Theory * 2 pawns a season, we'll need a MT of 3 or 4 to do this. We can choose a Large Leather bag for our purposes; that gives a +5 bonus to "create or transform within" which we can take advantage of if our MT is 5, so we'll presume it is. EDIT: Leather can handle 2 pawns, and large size is x4, so the resulting 8 pawns is well within the 5-7 we need to do this and it makes it reasonably sized for putting bigger things in.
To do this in a season, we need a MuTe lab total of either 82 or 122. If you're requiring requisites, then you use the lowest art of the An/Aq/Co/He/Te set. If not, then just MuTe. Working with the 82 total, let's just spitball some numbers, and presume we've also made this spell:
Mu 15 + Te 15 + Int 3 + MT 5 + Aura 5 + known spell 6 + S&M 5 = 54
If you've got an apprentice to help, we'll presume an additional + 6-- so 60.
If you're using Covenants and lab specializations, you could squeeze out a bit more for the space-- this is a rabbit hole of its own, so I'll just say +4 and leave the details to the reader. You could experiment, to gain a bit more, maybe + 9 on average (+3 risk & a +6 simple roll); there's a chance it all goes to hell, but this gets us to +73. If you've got a familiar with an intelligence of +3, and a MT of 6, you could get to the magic 82 target, but you'll be rolling on the risk chart.
Other options to adjust the lab total-- another helper in the lab, a MMF in something Mu/Te relevant, or-- and this is a big deal, get a laboratory text detailing the already created item. In this case, you'll just need to get past 41/61, and we can see that's feasible, even by yourself at MuTe30. The lab text could be traded from a Verditius, found in a ruined covenant's vaults, acquired in trade. Then you'd spend a season translating the text, and the next season creating the effect. This whole process could be an adventure followed by a busy Autumn and Winter for the creation. It depends on what you want to do more of. Does your wizard want to have adventures, or do you want to grind in the lab.
It's a lot for a bag of holding, but a fun exercise.