Auto Top Off via RO line?

[Funkytone]

I want to run my RO line over to the tank and design/buy (cheaply) some kind of an auto top off because I'm losing probably 1 - 2 gal a day and can't keep filling up the sump every day!

I've got the usual 1/4" line right now but from reading some recent posts on Gools thread about warming RO water, I think I'm going to go down to 3/8" and coil the line in the sump first.

What I'm mainly wondering about is what kind of auto top off systems are out there that are fairly inexpensive? or how about some good DIY's?

Thanks
-Tony