If you pass using the first void in the morning, then don't worry about subbing or diluting. You'll be fine to pass your random test. If not, I would go the sub route. Maintaining temperature is not as hard as it sounds. It can however be uncomfortable carrying the sample all day. If you can keep the urine close to your body(preferably touching) you'll having no problem keeping the temperature at 96-98 degrees. You'll want to find a flat travel size shampoo bottle, almost like a Elmers glue bottle, but smaller. Along with this, you'll need a handwarmer and a rubber band to affix the warmer to the bottle. You'll also need a temperature strip. You can find the ones that are made for use on baby's foreheads. They work great and you stick it right on the bottle. Most people prefer to wear two pairs of tighty whiteys and secure the bottle under the nether region. I prefer the strap the bottle to my inner thigh with an Ace bandage(not saying this is the way to do it, just what I prefer) but that is up to you. It's all quite simple, just practice as much as you can. You can fill your bottle with water, crouch it a get used to walking around with it, all while checking to see how fast your temperature is increasing.
Make sure you obtain your donor urine right before you go to work. Human urine starts to break down after 8 hours.