First, buy some molding mouth guard thing -- should have them at any department, drug or virtually any ecommerce/online market like Amazon. When you get the guard make sure to make an anchoring point to attach the resistance band/elastics/etc. to that will do the pulling for you.

Next step is something to anchor it to that reverses the force back against you -- something like a bendable bow with a natural tilt point centered along the length of it. You can basically buy a standard kids/toy bow and use that -- just need to "customize" it somewhat. If you use a kids bow or etc. you can attach the bottom part with another solid apparatus (can be anything as long as it's sturdy and not too flimsy) for the counter-force and balancing, while doing the same with the top part but making it counter from your forehead/frontal bone area. It may sound a bit complex, but it's actually not too out of this world -- just may take a few hours of experimenting once you've obtained all needed parts and/or concocted your own diagram mentally. What I thought was maybe connecting each end of a bow-like object (top and bottom) to other bows, the top part being just some flat, evened/non-slipping surface contact point for your forehead while the bottom just spreads or presses in to your sternum or chest-ish area.

You can get rubber bands for like 5 dollars or less and rig the guard easily -- the tougher part is of course the reverse force/bow setup but playing around should make it satisfactory. You can double up and go upper and lower dental arch, but probably not necessary. Wear it like any "real" face puller -- as long as you can and whenever acceptable during your day/night hours.

I think most other finished face pullers range in the hundreds of dollars range, but this can easily be done for well under 100 even if you don't cheap out much.