A Scriptable script that creates "invisible" widget backgrounds based on your iOS wallpaper, and then either uses them as a Scriptable widget background or exports to your camera roll.

mz_invisible_widget.js

// Variables used by Scriptable.
// These must be at the very top of the file. Do not edit.
// icon-color: deep-purple; icon-glyph: image;

// This widget was created by Max Zeryck @mzeryck

// Widgets are unique based on the name of the script.
const filename = Script.name() + ".jpg"
const files = FileManager.local()
const path = files.joinPath(files.documentsDirectory(), filename)

if (config.runsInWidget) {
  let widget = new ListWidget()
  widget.backgroundImage = files.readImage(path)
  
  // You can your own code here to add additional items to the "invisible" background of the widget.
  
  Script.setWidget(widget)
  Script.complete()

/*
 * The code below this comment is used to set up the invisible widget.
 * ===================================================================
 */
} else {
  
  // Determine if user has taken the screenshot.
  var message
  message = "Before you start, go to your home screen and enter wiggle mode. Scroll to the empty page on the far right and take a screenshot."
  let exitOptions = ["Continue","Exit to Take Screenshot"]
  let shouldExit = await generateAlert(message,exitOptions)
  if (shouldExit) return
  
  // Get screenshot and determine phone size.
  let img = await Photos.fromLibrary()
  let height = img.size.height
  let phone = phoneSizes()[height]
  if (!phone) {
    message = "It looks like you selected an image that isn't an iPhone screenshot, or your iPhone is not supported. Try again with a different image."
    await generateAlert(message,["OK"])
    return
  }
  
  // Prompt for widget size and position.
  message = "What size of widget are you creating?"
  let sizes = ["Small","Medium","Large"]
  let size = await generateAlert(message,sizes)
  let widgetSize = sizes[size]
  
  message = "What position will it be in?"
  message += (height == 1136 ? " (Note that your device only supports two rows of widgets, so the middle and bottom options are the same.)" : "")
  
  // Determine image crop based on phone size.
  let crop = { w: "", h: "", x: "", y: "" }
  if (widgetSize == "Small") {
    crop.w = phone.small
    crop.h = phone.small
    let positions = ["Top left","Top right","Middle left","Middle right","Bottom left","Bottom right"]
    let position = await generateAlert(message,positions)
    
    // Convert the two words into two keys for the phone size dictionary.
    let keys = positions[position].toLowerCase().split(' ')
    crop.y = phone[keys[0]]
    crop.x = phone[keys[1]]
    
  } else if (widgetSize == "Medium") {
    crop.w = phone.medium
    crop.h = phone.small
    
    // Medium and large widgets have a fixed x-value.
    crop.x = phone.left
    let positions = ["Top","Middle","Bottom"]
    let position = await generateAlert(message,positions)
    let key = positions[position].toLowerCase()
    crop.y = phone[key]
    
  } else if(widgetSize == "Large") {
    crop.w = phone.medium
    crop.h = phone.large
    crop.x = phone.left
    let positions = ["Top","Bottom"]
    let position = await generateAlert(message,positions)
    
    // Large widgets at the bottom have the "middle" y-value.
    crop.y = position ? phone.middle : phone.top
  }
  
  // Crop image and finalize the widget.
  let imgCrop = cropImage(img, new Rect(crop.x,crop.y,crop.w,crop.h))
  
  message = "Your widget background is ready. Would you like to use it in a Scriptable widget or export the image?"
  const exportPhotoOptions = ["Use in Scriptable","Export to Photos"]
  const exportPhoto = await generateAlert(message,exportPhotoOptions)
  
  if (exportPhoto) {
    Photos.save(imgCrop)
  } else {
    files.writeImage(path,imgCrop)
  }
  
  Script.complete()
}

// Generate an alert with the provided array of options.
async function generateAlert(message,options) {
  
  let alert = new Alert()
  alert.message = message
  
  for (const option of options) {
    alert.addAction(option)
  }
  
  let response = await alert.presentAlert()
  return response
}

// Crop an image into the specified rect.
function cropImage(img,rect) {
   
  let draw = new DrawContext()
  draw.size = new Size(rect.width, rect.height)
  
  draw.drawImageAtPoint(img,new Point(-rect.x, -rect.y))  
  return draw.getImage()
}

// Pixel sizes and positions for widgets on all supported phones.
function phoneSizes() {
  let phones = {	
	"2688": {
			"small":  507,
			"medium": 1080,
			"large":  1137,
			"left":  81,
			"right": 654,
			"top":    228,
			"middle": 858,
			"bottom": 1488
	},
	
	"1792": {
			"small":  338,
			"medium": 720,
			"large":  758,
			"left":  54,
			"right": 436,
			"top":    160,
			"middle": 580,
			"bottom": 1000
	},
	
	"2436": {
			"small":  465,
			"medium": 987,
			"large":  1035,
			"left":  69,
			"right": 591,
			"top":    213,
			"middle": 783,
			"bottom": 1353
	},
	
	"2208": {
			"small":  471,
			"medium": 1044,
			"large":  1071,
			"left":  99,
			"right": 672,
			"top":    114,
			"middle": 696,
			"bottom": 1278
	},
	
	"1334": {
			"small":  296,
			"medium": 642,
			"large":  648,
			"left":  54,
			"right": 400,
			"top":    60,
			"middle": 412,
			"bottom": 764
	},
	
	"1136": {
			"small":  282,
			"medium": 584,
			"large":  622,
			"left": 30,
			"right": 332,
			"top":  59,
			"middle": 399,
			"bottom": 399
	}
  }
  return phones
}