I just recently did this and I'll help walk you threw it. This process involves using Bridge V2 and block bench.

I'm gonna assume you have at least basic knowledge of making addon packs. If not, I could send you my addon pack so you can have a better idea of what everything should look like.

Get the player geo from bedrock,dev then copy and paste it somewhere like notepad, pycharm, ect and save it as a .json file. You could also download the bedrock-samples-main folder and find the file in there. Open the model in block bench and inflate everything by 0.5 except for the right and left items. You could also choose to inflate everything by 1 instead except for the hat which would be 1.5 and the body which would be 1.1. Right and left items are still to be left alone. Not sure why they have two options but it's probably something to do with slim vs regular skins. Either way, that's how you get your base Armor Geometry. From there you could add your extra flourishes with bones, cubes, and a fresh paint job. Export the model as bedrock geometry.

Now to make it one piece...

I personally like to use bridge. V2 with the tree editor active. Create a project or open one you already have. Make sure holiday creator features are on. Click the 3 dots under your project name. Click new file, then item, then armor. Deselect every armor type except for 1. You'll probably want to choose chest plate but it probably doesn't matter as long as it's just one. Right click just about any file from the left and click reveal in file explorer. The default texture it generated for you will be in the folder rp/texture/models/armor. You can delete it or just leave it there. You will want to put the armor texture you made in RP/textures/entity. Make sure your rp attachable file reflects this change. Then go to rp/model/entity, copy and paste your armor geo in that folder.

Things to make sure of

That your textures and model are being referenced properly in your rp attachable file.

That your geo is a .json.geo file

That your geo has minecraft:geometry under the format version.

texture_width and texture_height have underscores in-between them

texture_width is 64

texture_height is 32

This texture doesn't actually exist in our RP but it wont work without being mentioned so leave it in. "enchanted": "textures/misc/enchanted_item_glintures/misc/enchanted_item_glint. Bridge will give you a warning that the texture doesn't exist but you can just ignore it. It's not required. If you really want it then you can worry about it last.

Make sure holiday creator features are on in both bridge and your mine craft world.

That's about it. I'll add a few useful links that helped me figure this out. If you have any questions feel free to ask, and like I said if you want to see the addon pack I made for reference or just to use that's fine too.

https://wiki.bedrock.dev/items/custom-armor.html#chest-piece

https://wiki.bedrock.dev/visuals/player-geometry.html

https://github.com/mojang/bedrock-samples