As Modge Podge is used in all types of crafts, I found a tutorial over at
Instructables user nkeda14 on how to make cheap homemade Modge Podge interesting and something to try some time.
It sound fairly simple; just combine 1 1/2 c flour, 1/4 C of a sugar, 1/4 of oil (veg or olive) and 1 cup water over low to medium heat 5-10 mins stirring constantly, without bringing to a boil until the desired consistency is obtained,, adding flour or water as need or oil if looking for a higher varnish and store in a glass jar.
I don't know what the shelf life on this might be so if I try it, I would halve the recipe and try it on a small project.