What’s the best way to drain coolant on a 2wd 12 valve doesn’t look like a 5 gallon bucket will fit

A shallow pan. All you need is something that will hold 5 to 6 gallons. Wally world or most any parts store have has them. 

My 2 wheel drive.  Cheap harbor freight hand pump and pump into jugs, maybe 2 or 3 sucked out from topside.  Then my skinny drain pan for remaining fluid.  Not only do I spill less, I know how much i got out of cooling system.  I have yet to get out all.   

Everytime I crawl under my 2 wheel drive, I think about going on a diet??????

On my truck, I shove a full-size 5-gallon bucket under the drain cock and open its stops just short of the top of the bucket and slide it out. Then I remove the thermostat and get the garden hose and flush that block out really good. After the block is flushed out good I drain it again. Close the drain. Now the first gallon of coolant goes in straight. Being there is already water in the system. The remaining coolant is mixed 50/50. Continue to fill the system till the coolant is just about to spill out of the thermostat hole. Now install the NEW thermostat and upper hole and top off. No need to burp the system already do if you do it this way. 

 

I've been using strictly NAPA 375-190 thermostats for the life of the truck and still not had one fail! No need for fancy high priced Cummins thermostats...

 

https://www.napaonline.com/en/p/MRD375190