I like the topside creeper that was mentioned in the 3rd post by the OP. And along those lines a trick that works for me, especially working close to the firewall (or really deep into it) is 2" x 6" boards and a step stool. IIRC about 5' long & they span from my ARB bull bar to the cowl between windshield and firewall. They can be moved and strategically placed where you need them to lay on or support yourself. Cheap and effective.