Http_directory causing (404) not found error

I’m trying to copy a SQL Express installer (exe) which is 446MB to my hyper-v machine via the http_directory function of packer. I can confirm the correct environment variables are referencing the created http server:

hyperv-iso: PACKER_HTTP_ADDR

Here is my powershell script which executes as a provisioner:

      "scripts": [
      "type": "powershell"
[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls -bor [Net.SecurityProtocolType]::Tls11 -bor [Net.SecurityProtocolType]::Tls12
[string] $isoFile = "SQLEXPR_x64_ENU.exe"
[string] $source = "http://$ENV:PACKER_HTTP_ADDR/$isoFile"
[string] $destPath = "C:\Windows\Temp\$isoFile"
Write-Host "Attempting to copy from....."
$response = try { 
    (Invoke-WebRequest -Uri $source -OutFile $destPath -ErrorAction Stop).BaseResponse
} catch [System.Net.WebException] { 
    Write-Host "Completed Copy from HTTP"
    Write-Host "An exception was caught: $($_.Exception.Message)"

My script returns the error: ‘the remote server returned an error (404 not found)’.

My http_directory is set up off the root path that packer executes from:

  "builders": [
      "boot_wait": "6m",
      "communicator": "winrm",