Howdy
Ok, vivs: You can't have a viv that's too big, as long as there's plenty of options for your little one to hide in. They get freaked out if they find themselves in a massive (to them) area where they can't hide, as X predator might get it's claws/jaws/weird slimy bits on them. The bigger the viv, the more hides you should have in there (and the less you should expect to see your little one!) Heat mat should be big enough for them to coil themselves up over if they so choose, and should be closely regulated by a thermostat/ rheostat + thermometer partnership. You should have a thermometer in either case, but if you got with a rheostat rather than a thermostat it's even more essential.
Feeding enclosure: Smaller than their usual home (aka the viv) as you want to focus their attention on the food item. At the end of the day, this can be anything, as long as it fits the following requirements: there is an air flow (if you use a margarine tub punch holes in it), the snake feels secure enough to eat (some of my fussy eaters prefer opaque rather than transparent feed tubs), impregnable unless in the viv (I have mine in completely different tubs that aren't in the viv so must be escape proof, but if you put the feed tub in the locking viv this isn't so important) and containing no easily-eatable substrate (as in, don't put a little dish inside your viv with the pink on it as your snake will invariably drag the pink onto the substrate anyway. Pink covered in calci-sand=snake eating and getting blocked up with said sand). Ice-cream tub is fab for all of these as long as you punch some (non-escapeable) holes in it and have a paper towel for subsrate. (Even if they can't eat the paper towel, they still might poop. Scraping poop of an ice-cream tub = not fun)
If you're worried about melting plastic, you can always put something in-between; many here use tiles, cork, or newspaper. If you have a thermostat and put the probe directly above the heatmat it won't affect the temps but might avoid plastic-meltage.
Hope this helped!