there are literally hundreds of free designs on the interweb, we have built a half dozen at various times for amusement, the girls arent usually amused.
You dont need a big heat sink, thats why commercial smokers are just a sheet metal, its a slow cook over low heat with smoke.
My best was two clay flower pots with a grill shelf stuck in between, checking the product was really hard, I have one thats a 30gal steel trash can, its awesome, punched some holes and set rebar in to support the racks, the heater is a single burner hot plate from CTC for $18 that has a tin can of wood chips on it and a can of rum to infuse a great taste/smell, its totally getto, thats the appeal.
We made one out of an old refridgerator, it was huge and stupid, but the project cost $6.00 and we made 60lbs of ribs in one go.
simple is good, dont oversmoke the product, think flavour not ashtray. Its really hard to wreck a decent pork roast, start cheap and work your way up.