VS C# Project: How to organize 3rd Party DLLs used by DllImport

Question

I have a Visual Studio C# project that utilizes 3rd party C++ DLLs. Up until now there have been only 3 of these DLLs and so it hasn't bothered me to place these in the root folder of my project with the following properties:

Build Action: None
Copy to Output Directory: Copy Always

This results in the DLLs being copied to the project's /bin folder and all works fine - they do not appear in the published project's root folder. I reference these 3rd party DLLs like this:

public class MyDLL
{
    [DllImport("My_DLL.dll",
        EntryPoint = "?Unit@@YA?AUOutput@@UInput@@@Z",
        CallingConvention = CallingConvention.Cdecl)]
    public static extern MyDLLOutput Unit(MyDLLInput UnitInput);
}

Recently I've been provided with several dozens of additional DLLs. I do not reference these directly from within my app - these DLLs are used by one of the 3 DLLs I reference using DllImport().

Placing all of these support DLLs in the root folder of the project now becomes an issue with me as it clutters the project. I've tried placing them inside a folder structure such as:

/ExternalDLLs/DLL1/dll1.dll
/ExternalDLLs/DLL1/SupportDlls/1.dll
...
/ExternalDLLs/DLL1/SupportDlls/24.dll
/ExternalDLLs/DLL2/dll2.dll
/ExternalDLLs/DLL3/dll3.dll

This results in a published folder structure of:

/bin/ExternalDLLs/DLL1/dll1.dll
/bin/ExternalDLLs/DLL1/SupportDlls/1.dll
...
/bin/ExternalDLLs/DLL1/SupportDlls/24.dll
/bin/ExternalDLLs/DLL2/dll2.dll
/bin/ExternalDLLs/DLL3/dll3.dll

The published folder structure doesn't work with my DllImport() declaration. I presume this is because the DllImport() is expecting these to be directly underneath /bin/.

I don't mind a folder structure underneath /bin/ if necessary. I also don't mind if every DLL is copied to /bin/ without any folder structure - though I can't seem to find how to do that since the folder structure is also being copied to /bin/.

Can anyone point me on the best approach to overcome this? I don't see an attribute for DllImport() to specify a path. I'm sure this is a simple task, but I've never done it before. Hope I explained the problem well enough!

Thx.