You can always buy them pretty cheap at Todd's Cotumes or vintage ones on ebay. But if you are dead set on making your own... you can always use the search button up top. Just type: Indiana Jones Bag. There should be threads with info and images in them.
Edit:
Just did a search myself, and one of the first threads to pop up:
https://www.therpf.com/showthread.php?t=272951&highlight=INdiana+Jones+bag
There are measurements in the thread for the bag.