Coil cover's
You'll get a million different responses but personally I think it's better to just get the coil cover recommended for it, You should be able to get one for around $20.00 no matter what kind of coil you got. There's to many variables when you make alterations to the coil, The biggest being is does it VOID The warrenty. Sure the bed liner stuff is a COUPLE dollars cheaper but for the prices of coils these day's I will spend a couple more dollar's for the proper protection needed. Now with that being said you could always find some type of plastic sort of like a coffee can lid and poke a couple holes in it so you can attach it to the coil with zip ties to protect the bottom, That is if you have a spider type coil but PROTECT YOUR INVESTMENT and just buy one for it. It would suck to damage a $150.00 coil all because you tried to save $2.00 protecting it!! Some guy's will say " Oh, I've been detecting 30 yrs. and I never used one'' and most of them don't have that coil any more either but a lot of it has to do with swing style, Like do you run the coil on the ground are a couple inches off the ground. Hope that might help a little bit but I'll say it again BUY ONE!!!