Okay, let's start by saying.....there's a buncha things wrong with this scenario.
If you're only supplying the substrate, somewhere along the line, someone is gonna underbid you and most likely it will be one of the shops doing someone's sponsor board.
I would discuss with this person, they can keep everything under one roof by going with you and at the same time guarantee the quality of work going up at the track. Just ask him, do you want someone doing this in their cellar and another one digitally printed and have a huge difference ?? You want to keep uniformity and the high standards your shop can provide. You wont have that if you a have a dozen or two different shops supplying all the signs. Also, you can act as the go-between and have ALL the sponsors call you direct.... even if some of them have another sign shop. You very easily could land some new customers this way.
We just did this exact same scenario with a guy about 3 months ago and the first sign came in Tuesday. He pre-ordered a certain amount of blank (4x8s) and we'll just hafta print one side, but he knows the quality and turnaround will be spot on.
In the end, tell them you will supply a sign of your own for his sponsorship, as long as NO other shops are doing the work. You wouldn't wanna be associated with some hack's work if it goes up.
Also, you store them on end, not face to face, back to back or anything other than standing up. It'll take up less room, anyway. Then you don't need the edging.