Which mounting hardware`

Actually got some cabinet building done this weekend. After my Jasper jig mishap last time I had to build a homemade jig which seemed to work just fine. I will post some pictures after lunch--it seems my SD card has died. It was an all out clamp-o-rama.

My question is about driver mounting hardware. I bought the 1/4" kit from parts express but now I see that there is no way they are going to be long enough. With 2 layers of 3/4" MDF and some 1/4" plywood on the inside and the thickness of the cast frame on the RL-p15 the 1.5" bolts are going to come up short by quite a bit. What have you guys used in the past for a set-up like this?

Thanks