Good way of throwing in a single decoction is to start with a single infusion, pull about a third of the mash, boil for 30 minutes and add back. This will bring the mash temp to a good mash out point. Thick or thin part of mash can be used and don't have to worry about hitting a temp dead on.
I am currently getting my double decoction method down using a cooler for a mash tun. Just last night did one for a Rauchbier. Mashed in at 123*, infused boiling water to raise to 141*, pulled decoction, added back for a 154* rest, pulled second decoction, added back for a 165* mash out.