place sheets of paper or cardboard in your boot, transfer the contours onto it. then transfer that onto mdf - which will be the base/frame. remember its easier to cut off a little more than add back on.. ill assume you know what i mean by that.

still stand by the 'support blocks' idea.. they'll raise the height of the base/frame above the tyre and it fits in with you not wanting to drill into the chasis. then from there its a matter of your creativity..

kinda hard to put into words but what im picturing in my head is exactly what you're after.