Coolant tank

[Tailwind]

After a 45 mile run in the mountains, I noticed the low coolant warning. I check the tank and it was below the sensor.

Pulled the carpet and insulation. Coolant in truck.

Someone's been in there before...note loose bolt. Not too much coolant...

Cleaned everything off and filled reservoir to MIN with ~2/3 gal of distilled water.
Felt around backside seam and it's seeping. This morning I see a small puddle, no fluid from underneath, but seems originating from around the bolt and a thin line below the sensor. Never had a puddle under the car, so where'd so much coolant go?
Reading the articles on the DIY for the tank replacement, I don't want to tackle myself. What have members paid their Indies typically for the job?
TIA for your wisdom.