Conrad... I have had a go at leather bottoms whip stitched on both outside and inside several times, just as maxdaddy specified but had no luck coming up with a consistent process. So I couldn’t make several at a time for multiple customers. This cut into my profit margin big time. I have been doing 1/2” - 5/8” wood bottoms for a while now and it helps create a heavier, solid and easily water (beer) tight bottom. I use small 19s, 5/8” nails every 1.5” - 2” to keep it secured to the leather. Adhered around the inside edge of the wood. After sealing, the bottom becomes weighted and gives a very nice finished look if polyurethaned or Polyurethane is considered food safe, by the way.
Use whatever wood you like. I have used several but find that white oak, pin oak and mahogany are wonderful for weight. Poplar, cherry, locust create a beautiful color and pattern.
I have been using Envirotex on the inside only, for sealing if you have a customer that wants to enjoy both cold and hot beverage. But the downside is that it leaves a very high gloss finish and can be extremely messy during the sealing process.
If you want a more natural, “leather look”, use beeswax, paraffin wax or brewers pitch... pitch is a 50/50 mix of pine tar and beeswax. Very sticky when heated.