A quick and easy fat suit to make is two take two (2)
over-sized T-shirts (and by over-sized, I mean
over-sized for your actor) and -- using a sewing
mahine -- sew them together at the bottom hem.
Then sew a line from under each armpit to the
bottom hem again.

Stuff the space between the shirts with (washable)
filler. You can then sew in anatomical details (such
as pectorals or abdominals, forming pockets for the
filler). Once you've gotten the filler to the point that
pleases you, stitch the necklines together to contain
the filler. You may also want to stitch around the
arms -- this will allow you to pad the shoulders as
well.

Once completed, the final product will look
something like an umpire's chest pad.

Use heavy-duty t-shirts in your favorite color and toss
the whole thing into a washing machine once it gets
sweaty.

Stitching the arms will allow

 We used a fat suit of sorts, for 'Stepping Out' &  made up of foam & in the same manner of the T shirts, as suggested.
It worked well, but how comfortable it was I?m not sure.
Only it won our very first ?Phoscar Award' [Phoenix Oscar] For the fat suit & the trainer/wearer. Who has since resettled happily back in his New Mexico home town , with his Aussie wife, new son & of course the fat suit!