How can I weld a model correctly?

Hello, I’m trying to weld a model to make it a backpack and the results aren’t what I want, different parts of the model are different sizes and broken. I cant figure out how to weld the models correctly without them breaking.
image This is an example of the problem.
I put the backpack on a dummy at first and it still doesn’t look right.
I’ve tried different welds and nothing has worked yet.
My steps were ungrouping the model and welding each part individually.

Try making the model an accessory and then changing the attachment location. It’s what Booga Booga does and it works.

Changing the location of the attachment has done nothing, and it has been in an accessory. If this helps they are all mesh parts and maybe that is why they are deforming.

Normal welds are naturally going to put the backpack in the center of the part. Accessories are basically the last resort if this doesn’t work. Just in case if this helps, I’ll link an rbxm file with the Booga Booga bag.
God Bag From Booga Booga.rbxm (4.7 KB)

You’ll have to manually weld the model to the player’s back when they join the game in this case.

I was using a different type of weld and the god bag showed that I only needed regular welds.